diff --git a/src/OpenPolicyAgent.Ucast.Linq/QueryableExtensions.cs b/src/OpenPolicyAgent.Ucast.Linq/QueryableExtensions.cs
index 52051f5..4eb7cec 100644
--- a/src/OpenPolicyAgent.Ucast.Linq/QueryableExtensions.cs
+++ b/src/OpenPolicyAgent.Ucast.Linq/QueryableExtensions.cs
@@ -36,6 +36,11 @@ public static Type GetHigherPrecedenceNumericType(Type a, Type b)
         throw new InvalidOperationException($"Cannot determine precedence between {a} and {b}");
     }
 
+    public static bool IsGuidType(Type type)
+    {
+        return type == typeof(Guid);
+    }
+
     /// <summary>
     /// Builds a LINQ Lambda Expression from the UCAST tree, and then invokes
     /// it under a LINQ Where expression on some queryable data source.<br />
@@ -105,14 +110,18 @@ private static Expression BuildFieldExpression<T>(UCASTNode node, ParameterExpre
     /// </summary>
     /// <typeparam name="T">The LINQ</typeparam>
     /// <param name="node">Current UCAST node in the conditions tree.</param>
-    /// <param name="property">Derefered property lookup expression on the LINQ data source.</param>
+    /// <param name="property">Deferred property lookup expression on the LINQ data source.</param>
     /// <param name="mapper">Dictionary mapping UCAST property names to lambdas that generate LINQ Expressions.</param>
-    /// <returns>Result, a LINQ Expression (Usually a BinaryExpression).</returns>
+    /// <returns>Result, a LINQ BinaryExpression.</returns>
     /// <exception cref="ArgumentException">Thrown when arguments are of incompatible types.</exception>
-    private static Expression BuildFieldExpressionFromProperty<T>(UCASTNode node, Expression property, MappingConfiguration<T> mapper)
+    private static BinaryExpression BuildFieldExpressionFromProperty<T>(UCASTNode node, Expression property, MappingConfiguration<T> mapper)
     {
         Expression value = Expression.Constant(node.Value);
 
+        // If there is a type mismatch in an expression, it is usually from
+        // differing numeric types, or from things like a GUID vs String
+        // comparison. We try to make smart conversions here to ensure types
+        // are matched for the BinaryExpression result.
         Type lhsType = property.Type;
         Type rhsType = value.Type;
         if (lhsType != rhsType)
@@ -130,6 +139,21 @@ private static Expression BuildFieldExpressionFromProperty<T>(UCASTNode node, Ex
                     value = Expression.Convert(value, exprType);
                 }
             }
+            // Convert GUID strings automatically when the property is a Guid.
+            // Rego doesn't have native GUID typess, so it'll always be a
+            // GUID-formatted string that the policy is trying to match
+            // against the property.
+            else if (IsGuidType(lhsType) && rhsType == typeof(string))
+            {
+                if (Guid.TryParse(node.Value?.ToString(), out Guid guid))
+                {
+                    value = Expression.Constant(guid);
+                }
+                else
+                {
+                    throw new ArgumentException($"Expected a GUID-formatted string, but got '{node.Value}'");
+                }
+            }
         }
 
         // Switch expression:
@@ -164,7 +188,6 @@ private static Expression BuildFieldInExpression<T>(UCASTNode node, Expression p
         var eq = new UCASTNode("field", "eq", node.Field);
         var childValues = (List<object>)node.Value;
 
-
         // Iterate over all children, and determine highest-precedent type among
         // them. Convert LHS value if needed. RHS type conversions will happen
         // automatically during expression building later.
diff --git a/test/OpenPolicyAgent.Ucast.Linq.Tests/UnitTest.cs b/test/OpenPolicyAgent.Ucast.Linq.Tests/UnitTest.cs
index 0abd479..17bddf2 100644
--- a/test/OpenPolicyAgent.Ucast.Linq.Tests/UnitTest.cs
+++ b/test/OpenPolicyAgent.Ucast.Linq.Tests/UnitTest.cs
@@ -79,6 +79,7 @@ public static IEnumerable<object[]> EqTestData()
         yield return new object[] { new UCASTNode { Type = "field", Op = "eq", Field = "data.id", Value = (long)2 }, testdata.Where(d => d.Id == 2).ToList() };
         yield return new object[] { new UCASTNode { Type = "field", Op = "eq", Field = "data.flood_stage", Value = true }, testdata.Where(d => d.FloodStage).ToList() };
         yield return new object[] { new UCASTNode { Type = "field", Op = "eq", Field = "data.water_level_meters", Value = 5.8 }, testdata.Where(d => d.WaterLevelMeters == 5.8).ToList() };
+        yield return new object[] { new UCASTNode { Type = "field", Op = "eq", Field = "data.uuid", Value = "123e4567-e89b-12d3-a456-426614174000" }, testdata.Where(d => d.Uuid == new Guid("123e4567-e89b-12d3-a456-426614174000")).ToList() };
     }
 
     public static IEnumerable<object[]> NeTestData()
@@ -89,6 +90,7 @@ public static IEnumerable<object[]> NeTestData()
         yield return new object[] { new UCASTNode { Type = "field", Op = "ne", Field = "data.id", Value = (long)2 }, testdata.Where(d => d.Id != 2).ToList() };
         yield return new object[] { new UCASTNode { Type = "field", Op = "ne", Field = "data.flood_stage", Value = true }, testdata.Where(d => !d.FloodStage).ToList() };
         yield return new object[] { new UCASTNode { Type = "field", Op = "ne", Field = "data.water_level_meters", Value = 5.8 }, testdata.Where(d => d.WaterLevelMeters != 5.8).ToList() };
+        yield return new object[] { new UCASTNode { Type = "field", Op = "ne", Field = "data.uuid", Value = "123e4567-e89b-12d3-a456-426614174000" }, testdata.Where(d => d.Uuid != new Guid("123e4567-e89b-12d3-a456-426614174000")).ToList() };
     }
 
     public static IEnumerable<object[]> GtTestData()
@@ -128,6 +130,7 @@ public static IEnumerable<object[]> InTestData()
             yield return new object[] { new UCASTNode { Type = "field", Op = "in", Field = "data.id", Value = new List<object>() { (long)2, (long)5 } }, testdata.Where(d => new List<object>() { (long)2, (long)5 }.Contains((long)d.Id)).ToList() };
             yield return new object[] { new UCASTNode { Type = "field", Op = "in", Field = "data.flood_stage", Value = new List<object>() { true } }, testdata.Where(d => new List<object>() { true }.Contains(d.FloodStage)).ToList() };
             yield return new object[] { new UCASTNode { Type = "field", Op = "in", Field = "data.water_level_meters", Value = new List<object>() { 2.5, 5.8 } }, testdata.Where(d => new List<object>() { 2.5, 5.8 }.Contains(d.WaterLevelMeters)).ToList() };
+            yield return new object[] { new UCASTNode { Type = "field", Op = "in", Field = "data.uuid", Value = new List<object>() { "123e4567-e89b-12d3-a456-426614174000", "123e4567-e89b-12d3-a456-426614174001" } }, testdata.Where(d => new List<object>() { new Guid("123e4567-e89b-12d3-a456-426614174000"), new Guid("123e4567-e89b-12d3-a456-426614174001") }.Contains(d.Uuid)).ToList() };
         }
     }
 
@@ -140,6 +143,7 @@ public static IEnumerable<object[]> NinTestData()
             yield return new object[] { new UCASTNode { Type = "field", Op = "nin", Field = "data.id", Value = new List<object>() { (long)2, (long)5 } }, testdata.Where(d => !new List<object>() { (long)2, (long)5 }.Contains((long)d.Id)).ToList() };
             yield return new object[] { new UCASTNode { Type = "field", Op = "nin", Field = "data.flood_stage", Value = new List<object>() { true } }, testdata.Where(d => !new List<object>() { true }.Contains(d.FloodStage)).ToList() };
             yield return new object[] { new UCASTNode { Type = "field", Op = "nin", Field = "data.water_level_meters", Value = new List<object>() { 2.5, 5.8 } }, testdata.Where(d => !new List<object>() { 2.5, 5.8 }.Contains(d.WaterLevelMeters)).ToList() };
+            yield return new object[] { new UCASTNode { Type = "field", Op = "nin", Field = "data.uuid", Value = new List<object>() { "123e4567-e89b-12d3-a456-426614174000", "123e4567-e89b-12d3-a456-426614174001" } }, testdata.Where(d => !new List<object>() { new Guid("123e4567-e89b-12d3-a456-426614174000"), new Guid("123e4567-e89b-12d3-a456-426614174001") }.Contains(d.Uuid)).ToList() };
         }
     }
 }
@@ -503,6 +507,7 @@ public class UnitTestDataSource
     public class HydrologyData
     {
         public int Id { get; set; }
+        public Guid Uuid { get; set; }
         public string? Name { get; set; }
         public DateTime LastUpdated { get; set; }
         public bool FloodStage { get; set; }
@@ -513,11 +518,11 @@ public class HydrologyData
     public static List<HydrologyData> GetTestHydrologyData()
     {
         return [
-            new HydrologyData { Id = 1, Name = "River Alpha", LastUpdated = new DateTime(2024, 12, 10, 8, 30, 0), FloodStage = false, WaterLevelMeters = 2.5, FlowRateMinute = 100.5 },
-            new HydrologyData { Id = 2, Name = "Lake Beta", LastUpdated = new DateTime(2024, 12, 9, 15, 45, 0), FloodStage = true, WaterLevelMeters = 5.8, FlowRateMinute = null },
-            new HydrologyData { Id = 3, Name = "Stream Gamma", LastUpdated = new DateTime(2024, 12, 8, 12, 0, 0), FloodStage = false, WaterLevelMeters = 0.75, FlowRateMinute = 25.3 },
-            new HydrologyData { Id = 4, Name = "Reservoir Delta", LastUpdated = new DateTime(2024, 12, 7, 9, 15, 0), FloodStage = false, WaterLevelMeters = 15.2, FlowRateMinute = 500.0 },
-            new HydrologyData { Id = 5, Name = null, LastUpdated = new DateTime(2024, 12, 6, 18, 30, 0), FloodStage = true, WaterLevelMeters = 3.1, FlowRateMinute = 75.8 }
+            new HydrologyData { Id = 1, Uuid = new Guid("123e4567-e89b-12d3-a456-426614174000"), Name = "River Alpha", LastUpdated = new DateTime(2024, 12, 10, 8, 30, 0), FloodStage = false, WaterLevelMeters = 2.5, FlowRateMinute = 100.5 },
+            new HydrologyData { Id = 2, Uuid = new Guid("123e4567-e89b-12d3-a456-426614174001"), Name = "Lake Beta", LastUpdated = new DateTime(2024, 12, 9, 15, 45, 0), FloodStage = true, WaterLevelMeters = 5.8, FlowRateMinute = null },
+            new HydrologyData { Id = 3, Uuid = new Guid("123e4567-e89b-12d3-a456-426614174002"), Name = "Stream Gamma", LastUpdated = new DateTime(2024, 12, 8, 12, 0, 0), FloodStage = false, WaterLevelMeters = 0.75, FlowRateMinute = 25.3 },
+            new HydrologyData { Id = 4, Uuid = new Guid("123e4567-e89b-12d3-a456-426614174003"), Name = "Reservoir Delta", LastUpdated = new DateTime(2024, 12, 7, 9, 15, 0), FloodStage = false, WaterLevelMeters = 15.2, FlowRateMinute = 500.0 },
+            new HydrologyData { Id = 5, Uuid = new Guid("123e4567-e89b-12d3-a456-426614174004"), Name = null, LastUpdated = new DateTime(2024, 12, 6, 18, 30, 0), FloodStage = true, WaterLevelMeters = 3.1, FlowRateMinute = 75.8 }
         ];
     }