diff --git a/packages/core/package.json b/packages/core/package.json
new file mode 100644
index 0000000..8a97a80
--- /dev/null
+++ b/packages/core/package.json
@@ -0,0 +1,15 @@
+{
+  "name": "@understand-anything/core",
+  "version": "0.1.0",
+  "type": "module",
+  "main": "dist/index.js",
+  "types": "dist/index.d.ts",
+  "scripts": {
+    "build": "tsc",
+    "test": "vitest run"
+  },
+  "devDependencies": {
+    "typescript": "^5.7.0",
+    "vitest": "^3.1.0"
+  }
+}
diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
new file mode 100644
index 0000000..a0c4ebf
--- /dev/null
+++ b/packages/core/src/index.ts
@@ -0,0 +1 @@
+export * from "./types.js";
diff --git a/packages/core/src/types.test.ts b/packages/core/src/types.test.ts
new file mode 100644
index 0000000..e155b8f
--- /dev/null
+++ b/packages/core/src/types.test.ts
@@ -0,0 +1,117 @@
+import { describe, it, expect } from "vitest";
+import type { KnowledgeGraph, GraphNode, GraphEdge } from "./types.js";
+
+describe("KnowledgeGraph types", () => {
+  it("should create a valid empty KnowledgeGraph", () => {
+    const graph: KnowledgeGraph = {
+      version: "1.0.0",
+      project: {
+        name: "test-project",
+        languages: [],
+        frameworks: [],
+        description: "A test project",
+        analyzedAt: new Date().toISOString(),
+        gitCommitHash: "abc123",
+      },
+      nodes: [],
+      edges: [],
+      layers: [],
+      tour: [],
+    };
+
+    expect(graph.version).toBe("1.0.0");
+    expect(graph.project.name).toBe("test-project");
+    expect(graph.nodes).toHaveLength(0);
+    expect(graph.edges).toHaveLength(0);
+    expect(graph.layers).toHaveLength(0);
+    expect(graph.tour).toHaveLength(0);
+  });
+
+  it("should create valid GraphNodes with all fields", () => {
+    const fileNode: GraphNode = {
+      id: "node-1",
+      type: "file",
+      name: "index.ts",
+      filePath: "src/index.ts",
+      lineRange: [1, 50],
+      summary: "Entry point of the application",
+      tags: ["entry", "typescript"],
+      complexity: "simple",
+      languageNotes: "Uses ES module syntax",
+    };
+
+    expect(fileNode.id).toBe("node-1");
+    expect(fileNode.type).toBe("file");
+    expect(fileNode.filePath).toBe("src/index.ts");
+    expect(fileNode.lineRange).toEqual([1, 50]);
+    expect(fileNode.tags).toContain("entry");
+    expect(fileNode.complexity).toBe("simple");
+
+    const functionNode: GraphNode = {
+      id: "node-2",
+      type: "function",
+      name: "processData",
+      filePath: "src/utils.ts",
+      lineRange: [10, 25],
+      summary: "Processes raw data into structured format",
+      tags: ["utility", "data"],
+      complexity: "moderate",
+    };
+
+    expect(functionNode.type).toBe("function");
+    expect(functionNode.complexity).toBe("moderate");
+    expect(functionNode.languageNotes).toBeUndefined();
+
+    const classNode: GraphNode = {
+      id: "node-3",
+      type: "class",
+      name: "DataStore",
+      summary: "Manages application state",
+      tags: ["state", "core"],
+      complexity: "complex",
+    };
+
+    expect(classNode.type).toBe("class");
+    expect(classNode.filePath).toBeUndefined();
+    expect(classNode.lineRange).toBeUndefined();
+  });
+
+  it("should create valid GraphEdges with weight bounds", () => {
+    const edge: GraphEdge = {
+      source: "node-1",
+      target: "node-2",
+      type: "imports",
+      direction: "forward",
+      description: "index.ts imports processData from utils.ts",
+      weight: 0.8,
+    };
+
+    expect(edge.source).toBe("node-1");
+    expect(edge.target).toBe("node-2");
+    expect(edge.type).toBe("imports");
+    expect(edge.direction).toBe("forward");
+    expect(edge.weight).toBeGreaterThanOrEqual(0);
+    expect(edge.weight).toBeLessThanOrEqual(1);
+
+    const minWeightEdge: GraphEdge = {
+      source: "node-1",
+      target: "node-3",
+      type: "related",
+      direction: "bidirectional",
+      weight: 0,
+    };
+
+    expect(minWeightEdge.weight).toBe(0);
+    expect(minWeightEdge.description).toBeUndefined();
+
+    const maxWeightEdge: GraphEdge = {
+      source: "node-2",
+      target: "node-3",
+      type: "calls",
+      direction: "forward",
+      weight: 1,
+    };
+
+    expect(maxWeightEdge.weight).toBe(1);
+  });
+});
diff --git a/packages/core/src/types.ts b/packages/core/src/types.ts
new file mode 100644
index 0000000..d9884e3
--- /dev/null
+++ b/packages/core/src/types.ts
@@ -0,0 +1,103 @@
+// Edge types (17 total in 5 categories: Structural, Behavioral, Data flow, Dependencies, Semantic)
+export type EdgeType =
+  | "imports" | "exports" | "contains" | "inherits" | "implements"  // Structural
+  | "calls" | "subscribes" | "publishes" | "middleware"              // Behavioral
+  | "reads_from" | "writes_to" | "transforms" | "validates"         // Data flow
+  | "depends_on" | "tested_by" | "configures"                       // Dependencies
+  | "related" | "similar_to";                                        // Semantic
+
+// GraphNode with 5 types: file, function, class, module, concept
+export interface GraphNode {
+  id: string;
+  type: "file" | "function" | "class" | "module" | "concept";
+  name: string;
+  filePath?: string;
+  lineRange?: [number, number];
+  summary: string;
+  tags: string[];
+  complexity: "simple" | "moderate" | "complex";
+  languageNotes?: string;
+}
+
+// GraphEdge with rich relationship modeling
+export interface GraphEdge {
+  source: string;
+  target: string;
+  type: EdgeType;
+  direction: "forward" | "backward" | "bidirectional";
+  description?: string;
+  weight: number; // 0-1
+}
+
+// Layer (logical grouping)
+export interface Layer {
+  id: string;
+  name: string;
+  description: string;
+  nodeIds: string[];
+}
+
+// TourStep (for learn mode)
+export interface TourStep {
+  order: number;
+  title: string;
+  description: string;
+  nodeIds: string[];
+  languageLesson?: string;
+}
+
+// ProjectMeta
+export interface ProjectMeta {
+  name: string;
+  languages: string[];
+  frameworks: string[];
+  description: string;
+  analyzedAt: string;
+  gitCommitHash: string;
+}
+
+// Root KnowledgeGraph
+export interface KnowledgeGraph {
+  version: string;
+  project: ProjectMeta;
+  nodes: GraphNode[];
+  edges: GraphEdge[];
+  layers: Layer[];
+  tour: TourStep[];
+}
+
+// AnalysisMeta (for persistence)
+export interface AnalysisMeta {
+  lastAnalyzedAt: string;
+  gitCommitHash: string;
+  version: string;
+  analyzedFiles: number;
+}
+
+// Plugin interfaces
+export interface StructuralAnalysis {
+  functions: Array<{ name: string; lineRange: [number, number]; params: string[]; returnType?: string }>;
+  classes: Array<{ name: string; lineRange: [number, number]; methods: string[]; properties: string[] }>;
+  imports: Array<{ source: string; specifiers: string[]; lineNumber: number }>;
+  exports: Array<{ name: string; lineNumber: number }>;
+}
+
+export interface ImportResolution {
+  source: string;
+  resolvedPath: string;
+  specifiers: string[];
+}
+
+export interface CallGraphEntry {
+  caller: string;
+  callee: string;
+  lineNumber: number;
+}
+
+export interface AnalyzerPlugin {
+  name: string;
+  languages: string[];
+  analyzeFile(filePath: string, content: string): StructuralAnalysis;
+  resolveImports(filePath: string, content: string): ImportResolution[];
+  extractCallGraph?(filePath: string, content: string): CallGraphEntry[];
+}
diff --git a/packages/core/tsconfig.json b/packages/core/tsconfig.json
new file mode 100644
index 0000000..a086b14
--- /dev/null
+++ b/packages/core/tsconfig.json
@@ -0,0 +1,8 @@
+{
+  "extends": "../../tsconfig.json",
+  "compilerOptions": {
+    "outDir": "dist",
+    "rootDir": "src"
+  },
+  "include": ["src"]
+}
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index c4b6858..e2e9acb 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -15,6 +15,15 @@ importers:
         specifier: ^3.1.0
         version: 3.2.4
 
+  packages/core:
+    devDependencies:
+      typescript:
+        specifier: ^5.7.0
+        version: 5.9.3
+      vitest:
+        specifier: ^3.1.0
+        version: 3.2.4
+
 packages:
 
   '@esbuild/aix-ppc64@0.27.4':