diff --git a/src/index.ts b/src/index.ts
index 8aa7fa2..4cf9b90 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -28,14 +28,14 @@ import { NoTTYReport } from './log/no-tty-report';
 
 const args = mri(process.argv.slice(2), {
   alias: { p: 'project', c: 'config' },
-  default: { project: process.cwd(), config: '', silent: false },
+  default: { project: process.cwd(), config: '', silent: false, compare: '' },
 });
 
 /**
  * Read the configuration from the specified project, validate it, perform requested compression, and report the results.
  */
 (async function () {
-  const { project: projectPath, silent, config: requestedConfig } = args;
+  const { project: projectPath, silent, config: requestedConfig, compare } = args;
   const conditions = [Project, Config];
   const context: Context = {
     projectPath,
@@ -47,6 +47,7 @@ const args = mri(process.argv.slice(2), {
     compressed: new Map(),
     // Stores the basis of comparison.
     comparison: new Map(),
+    comparisonPath: compare,
     fileModifier: null,
     fileContents: new Map(),
   };
diff --git a/src/validation/Compare.ts b/src/validation/Compare.ts
new file mode 100644
index 0000000..f759f4b
--- /dev/null
+++ b/src/validation/Compare.ts
@@ -0,0 +1,73 @@
+/**
+ * Copyright 2021 The AMP HTML Authors. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS-IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { ConditionFunction, Context, SizeMap } from './Condition';
+import { isDirectory, isFile, readFile } from '../helpers/fs';
+import { resolve, dirname } from 'path';
+import { MakeError } from '../log/helpers/error';
+
+/**
+ * Ensure context contains a valid project directory and `package.json` inside.
+ * @param context
+ */
+const Compare: ConditionFunction = (context: Context) => {
+  return async function () {
+    if (context.comparisonPath !== '') {
+      // The comparison path was specified up front
+      if (!(await isFile(context.comparisonPath))) {
+        return MakeError(
+          `comparison path specified '${context.comparisonPath}' doesn't exist, is this a valid comparison?`,
+        );
+      }
+
+      const comparisonFileContent = await readFile(context.comparisonPath);
+      if (comparisonFileContent === null) {
+        return MakeError(
+          `comparison path specified '${context.comparisonPath}' appears invalid, does it have contents?`,
+        );
+      }
+
+      context.comparison = new Map(JSON.parse(comparisonFileContent)) as SizeMap;
+      // TODO: Likely need to do some validation on the contents to ensure its a SizeMap structure.
+    }
+
+    return null;
+    // if (context.packagePath !== '') {
+    //   // The package path was specified up front, its likely not a package.json
+    //   if (!(await isFile(context.packagePath))) {
+    //     return MakeError(`config specified '${context.packagePath}' doesn't exist, is this a valid project?`);
+    //   }
+    //   context.projectPath = dirname(context.packagePath);
+    //   return null;
+    // }
+
+    // const projectPath: string = resolve(context.projectPath);
+    // if (!(await isDirectory(projectPath))) {
+    //   return MakeError(`project specified '${context.projectPath}' doesn't exist, is this a valid project?`);
+    // }
+
+    // const packagePath = resolve(context.projectPath, 'package.json');
+    // if (!(await isFile(packagePath))) {
+    //   return MakeError(`Missing '${packagePath}', is this a valid project?`);
+    // }
+
+    // context.projectPath = projectPath;
+    // context.packagePath = packagePath;
+    // return null;
+  };
+};
+
+export default Compare;
diff --git a/src/validation/Condition.ts b/src/validation/Condition.ts
index b22c2f8..76168ea 100644
--- a/src/validation/Condition.ts
+++ b/src/validation/Condition.ts
@@ -60,6 +60,8 @@ export interface Context {
   compressed: SizeMap;
   // Stores the basis of comparison.
   comparison: SizeMap;
+  // The filepath of a comparison map
+  comparisonPath: string;
   // Allows the API to specify a method that alters content before analysis.
   fileModifier: FileModifier;
   // Stores the contents of files, to avoid reading from disk per compression type.