What is the testing pyramid?

The testing pyramid suggests a distribution of test types: Unit tests (base, most numerous) - fast, isolated, test individual components. Integration tests (middle) - test component interactions, databases, APIs. E2E/UI tests (top, fewest) - test complete user flows, slowest and most brittle. More unit tests because they're fast and cheap; fewer E2E tests because they're slow and expensive. Some prefer testing trophy or honeycomb shapes for different contexts.