diff --git a/BEFORE_SEND.md b/BEFORE_SEND.md
new file mode 100644
index 00000000..a3696d3d
--- /dev/null
+++ b/BEFORE_SEND.md
@@ -0,0 +1,237 @@
+# Before Send Hook
+
+The `before_send` parameter allows you to modify or filter events before they are sent to PostHog. This is useful for:
+
+- **Privacy**: Removing or masking sensitive data (PII)
+- **Filtering**: Dropping unwanted events (test events, internal users, etc.)
+- **Enhancement**: Adding custom properties to all events
+- **Transformation**: Modifying event names or property formats
+
+## Basic Usage
+
+```python
+import posthog
+from typing import Optional, Dict, Any
+
+def my_before_send(event: Dict[str, Any]) -> Optional[Dict[str, Any]]:
+    """
+    Process event before sending to PostHog.
+    
+    Args:
+        event: The event dictionary containing 'event', 'distinct_id', 'properties', etc.
+    
+    Returns:
+        Modified event dictionary to send, or None to drop the event
+    """
+    # Your processing logic here
+    return event
+
+# Initialize client with before_send hook
+client = posthog.Client(
+    api_key="your-project-api-key",
+    before_send=my_before_send
+)
+```
+
+## Common Use Cases
+
+### 1. Filter Out Events
+
+```python
+from typing import Optional, Any
+
+def filter_events_by_property_or_event_name(event: dict[str, Any]) -> Optional[dict[str, Any]]:
+    """Drop events from internal users or test environments."""
+    properties = event.get("properties", {})
+    
+    # Choose some property from your events
+    event_source = properties.get("event_source", "")
+    if event_source.endswith("internal"):
+        return None  # Drop the event
+    
+    # Filter out test events
+    if event.get("event") == "test_event":
+        return None
+    
+    return event
+```
+
+### 2. Remove/Mask PII Data
+
+```python
+from typing import Optional, Any
+
+def scrub_pii(event: dict[str, Any]) -> Optional[dict[str, Any]]:
+    """Remove or mask personally identifiable information."""
+    properties = event.get("properties", {})
+    
+    # Mask email but keep domain for analytics
+    if "email" in properties:
+        email = properties["email"]
+        if "@" in email:
+            domain = email.split("@")[1]
+            properties["email"] = f"***@{domain}"
+        else:
+            properties["email"] = "***"
+    
+    # Remove sensitive fields entirely
+    sensitive_fields = ["my_business_info", "secret_things"]
+    for field in sensitive_fields:
+        properties.pop(field, None)
+    
+    return event
+```
+
+### 3. Add Custom Properties
+
+```python
+from typing import Optional, Any
+
+from datetime import datetime
+from typing import Optional, Any
+
+def add_context(event: dict[str, Any]) -> Optional[dict[str, Any]]:
+    """Add custom properties to all events."""
+    if "properties" not in event:
+        event["properties"] = {}
+    
+    event["properties"].update({
+        "app_version": "2.1.0",
+        "environment": "production", 
+        "processed_at": datetime.now().isoformat()
+    })
+    
+    return event
+```
+
+### 4. Transform Event Names
+
+```python
+from typing import Optional, Any
+
+def normalize_event_names(event: dict[str, Any]) -> Optional[dict[str, Any]]:
+    """Convert event names to a consistent format."""
+    original_event = event.get("event")
+    if original_event:
+        # Convert to snake_case
+        normalized = original_event.lower().replace(" ", "_").replace("-", "_")
+        event["event"] = f"app_{normalized}"
+    
+    return event
+```
+
+### 5. Log and drop in "dev" mode
+
+When running in local dev often, you want to log but drop all events
+
+
+```python
+from typing import Optional, Any
+
+def log_and_drop_all(event: dict[str, Any]) -> Optional[dict[str, Any]]:
+    """Convert event names to a consistent format."""
+    print(event)
+    
+    return None
+```
+
+### 6. Combined Processing
+
+```python
+from typing import Optional, Any
+
+def comprehensive_processor(event: dict[str, Any]) -> Optional[dict[str, Any]]:
+    """Apply multiple transformations in sequence."""
+    
+    # Step 1: Filter unwanted events
+    if should_drop_event(event):
+        return None
+    
+    # Step 2: Scrub PII
+    event = scrub_pii(event)
+    
+    # Step 3: Add context
+    event = add_context(event)
+    
+    # Step 4: Normalize names
+    event = normalize_event_names(event)
+    
+    return event
+
+def should_drop_event(event: dict[str, Any]) -> bool:
+    """Determine if event should be dropped."""
+    # Your filtering logic
+    return False
+```
+
+## Error Handling
+
+If your `before_send` function raises an exception, PostHog will:
+
+1. Log the error 
+2. Continue with the original, unmodified event
+3. Not crash your application
+
+```python
+from typing import Optional, Any
+
+def risky_before_send(event: dict[str, Any]) -> Optional[dict[str, Any]]:
+    # If this raises an exception, the original event will be sent
+    risky_operation()
+    return event
+```
+
+## Complete Example
+
+```python
+import posthog
+from typing import Optional, Any
+import re
+
+def production_before_send(event: dict[str, Any]) -> Optional[dict[str, Any]]:
+    try:
+        properties = event.get("properties", {})
+        
+        # 1. Filter out bot traffic
+        user_agent = properties.get("$user_agent", "")
+        if re.search(r'bot|crawler|spider', user_agent, re.I):
+            return None
+        
+        # 2. Filter out internal traffic  
+        ip = properties.get("$ip", "")
+        if ip.startswith("192.168.") or ip.startswith("10."):
+            return None
+        
+        # 3. Scrub email PII but keep domain
+        if "email" in properties:
+            email = properties["email"]
+            if "@" in email:
+                domain = email.split("@")[1]
+                properties["email"] = f"***@{domain}"
+        
+        # 4. Add custom context
+        properties.update({
+            "app_version": "1.0.0",
+            "build_number": "123"
+        })
+        
+        # 5. Normalize event name
+        if event.get("event"):
+            event["event"] = event["event"].lower().replace(" ", "_")
+        
+        return event
+        
+    except Exception as e:
+        # Log error but don't crash
+        print(f"Error in before_send: {e}")
+        return event  # Return original event on error
+
+# Usage
+client = posthog.Client(
+    api_key="your-api-key",
+    before_send=production_before_send
+)
+
+# All events will now be processed by your before_send function
+client.capture("user_123", "Page View", {"url": "/home"})
+```
\ No newline at end of file
diff --git a/CHANGELOG.md b/CHANGELOG.md
index ed44c797..31fd5ffc 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 4.5.0- 2025-06-09
+
+- feat: add before_send callback (#249)
+
 ## 4.4.2- 2025-06-09
 
 - empty point release to fix release automation
diff --git a/posthog/client.py b/posthog/client.py
index 3bd68ad9..a6ce4423 100644
--- a/posthog/client.py
+++ b/posthog/client.py
@@ -144,6 +144,7 @@ def __init__(
         exception_autocapture_integrations=None,
         project_root=None,
         privacy_mode=False,
+        before_send=None,
     ):
         self.queue = queue.Queue(max_queue_size)
 
@@ -199,6 +200,15 @@ def __init__(
         else:
             self.log.setLevel(logging.WARNING)
 
+        if before_send is not None:
+            if callable(before_send):
+                self.before_send = before_send
+            else:
+                self.log.warning("before_send is not callable, it will be ignored")
+                self.before_send = None
+        else:
+            self.before_send = None
+
         if self.enable_exception_autocapture:
             self.exception_capture = ExceptionCapture(
                 self, integrations=self.exception_autocapture_integrations
@@ -744,6 +754,18 @@ def _enqueue(self, msg, disable_geoip):
         msg["distinct_id"] = stringify_id(msg.get("distinct_id", None))
 
         msg = clean(msg)
+
+        if self.before_send:
+            try:
+                modified_msg = self.before_send(msg)
+                if modified_msg is None:
+                    self.log.debug("Event dropped by before_send callback")
+                    return True, None
+                msg = modified_msg
+            except Exception as e:
+                self.log.exception(f"Error in before_send callback: {e}")
+                # Continue with the original message if callback fails
+
         self.log.debug("queueing: %s", msg)
 
         # if send is False, return msg as if it was successfully queued
diff --git a/posthog/test/test_before_send.py b/posthog/test/test_before_send.py
new file mode 100644
index 00000000..e081e0d1
--- /dev/null
+++ b/posthog/test/test_before_send.py
@@ -0,0 +1,171 @@
+import unittest
+
+import mock
+
+from posthog.client import Client
+from posthog.test.test_utils import FAKE_TEST_API_KEY
+
+
+class TestClient(unittest.TestCase):
+    @classmethod
+    def setUpClass(cls):
+        # This ensures no real HTTP POST requests are made
+        cls.client_post_patcher = mock.patch("posthog.client.batch_post")
+        cls.consumer_post_patcher = mock.patch("posthog.consumer.batch_post")
+        cls.client_post_patcher.start()
+        cls.consumer_post_patcher.start()
+
+    @classmethod
+    def tearDownClass(cls):
+        cls.client_post_patcher.stop()
+        cls.consumer_post_patcher.stop()
+
+    def set_fail(self, e, batch):
+        """Mark the failure handler"""
+        print("FAIL", e, batch)  # noqa: T201
+        self.failed = True
+
+    def setUp(self):
+        self.failed = False
+        self.client = Client(FAKE_TEST_API_KEY, on_error=self.set_fail)
+
+    def test_before_send_callback_modifies_event(self):
+        """Test that before_send callback can modify events."""
+        processed_events = []
+
+        def my_before_send(event):
+            processed_events.append(event.copy())
+            if "properties" not in event:
+                event["properties"] = {}
+            event["properties"]["processed_by_before_send"] = True
+            return event
+
+        client = Client(
+            FAKE_TEST_API_KEY, on_error=self.set_fail, before_send=my_before_send
+        )
+        success, msg = client.capture("user1", "test_event", {"original": "value"})
+
+        self.assertTrue(success)
+        self.assertEqual(msg["properties"]["processed_by_before_send"], True)
+        self.assertEqual(msg["properties"]["original"], "value")
+        self.assertEqual(len(processed_events), 1)
+        self.assertEqual(processed_events[0]["event"], "test_event")
+
+    def test_before_send_callback_drops_event(self):
+        """Test that before_send callback can drop events by returning None."""
+
+        def drop_test_events(event):
+            if event.get("event") == "test_drop_me":
+                return None
+            return event
+
+        client = Client(
+            FAKE_TEST_API_KEY, on_error=self.set_fail, before_send=drop_test_events
+        )
+
+        # Event should be dropped
+        success, msg = client.capture("user1", "test_drop_me")
+        self.assertTrue(success)
+        self.assertIsNone(msg)
+
+        # Event should go through
+        success, msg = client.capture("user1", "keep_me")
+        self.assertTrue(success)
+        self.assertIsNotNone(msg)
+        self.assertEqual(msg["event"], "keep_me")
+
+    def test_before_send_callback_handles_exceptions(self):
+        """Test that exceptions in before_send don't crash the client."""
+
+        def buggy_before_send(event):
+            raise ValueError("Oops!")
+
+        client = Client(
+            FAKE_TEST_API_KEY, on_error=self.set_fail, before_send=buggy_before_send
+        )
+        success, msg = client.capture("user1", "robust_event")
+
+        # Event should still be sent despite the exception
+        self.assertTrue(success)
+        self.assertIsNotNone(msg)
+        self.assertEqual(msg["event"], "robust_event")
+
+    def test_before_send_callback_works_with_all_event_types(self):
+        """Test that before_send works with capture, identify, set, etc."""
+
+        def add_marker(event):
+            if "properties" not in event:
+                event["properties"] = {}
+            event["properties"]["marked"] = True
+            return event
+
+        client = Client(
+            FAKE_TEST_API_KEY, on_error=self.set_fail, before_send=add_marker
+        )
+
+        # Test capture
+        success, msg = client.capture("user1", "event")
+        self.assertTrue(success)
+        self.assertTrue(msg["properties"]["marked"])
+
+        # Test identify
+        success, msg = client.identify("user1", {"trait": "value"})
+        self.assertTrue(success)
+        self.assertTrue(msg["properties"]["marked"])
+
+        # Test set
+        success, msg = client.set("user1", {"prop": "value"})
+        self.assertTrue(success)
+        self.assertTrue(msg["properties"]["marked"])
+
+        # Test page
+        success, msg = client.page("user1", "https://example.com")
+        self.assertTrue(success)
+        self.assertTrue(msg["properties"]["marked"])
+
+    def test_before_send_callback_disabled_when_none(self):
+        """Test that client works normally when before_send is None."""
+        client = Client(FAKE_TEST_API_KEY, on_error=self.set_fail, before_send=None)
+        success, msg = client.capture("user1", "normal_event")
+
+        self.assertTrue(success)
+        self.assertIsNotNone(msg)
+        self.assertEqual(msg["event"], "normal_event")
+
+    def test_before_send_callback_pii_scrubbing_example(self):
+        """Test a realistic PII scrubbing use case."""
+
+        def scrub_pii(event):
+            properties = event.get("properties", {})
+
+            # Mask email but keep domain
+            if "email" in properties:
+                email = properties["email"]
+                if "@" in email:
+                    domain = email.split("@")[1]
+                    properties["email"] = f"***@{domain}"
+                else:
+                    properties["email"] = "***"
+
+            # Remove credit card
+            properties.pop("credit_card", None)
+
+            return event
+
+        client = Client(
+            FAKE_TEST_API_KEY, on_error=self.set_fail, before_send=scrub_pii
+        )
+        success, msg = client.capture(
+            "user1",
+            "form_submit",
+            {
+                "email": "user@example.com",
+                "credit_card": "1234-5678-9012-3456",
+                "form_name": "contact",
+            },
+        )
+
+        self.assertTrue(success)
+        self.assertEqual(msg["properties"]["email"], "***@example.com")
+        self.assertNotIn("credit_card", msg["properties"])
+        self.assertEqual(msg["properties"]["form_name"], "contact")
diff --git a/posthog/types.py b/posthog/types.py
index 4c3796ea..97de99a1 100644
--- a/posthog/types.py
+++ b/posthog/types.py
@@ -1,9 +1,13 @@
 import json
 from dataclasses import dataclass
-from typing import Any, List, Optional, TypedDict, Union, cast
+from typing import Any, Callable, List, Optional, TypedDict, Union, cast
 
 FlagValue = Union[bool, str]
 
+# Type alias for the before_send callback function
+# Takes an event dictionary and returns the modified event or None to drop it
+BeforeSendCallback = Callable[[dict[str, Any]], Optional[dict[str, Any]]]
+
 
 @dataclass(frozen=True)
 class FlagReason:
diff --git a/posthog/version.py b/posthog/version.py
index d262c5ba..b996a2ab 100644
--- a/posthog/version.py
+++ b/posthog/version.py
@@ -1,4 +1,4 @@
-VERSION = "4.4.2"
+VERSION = "4.5.0"
 
 if __name__ == "__main__":
     print(VERSION, end="")  # noqa: T201