The final A-level year (ages 17–18) turns to the deep theory that ties the whole subject together: how to measure and design efficient algorithms, the networks and databases that run the real world, the cryptography that keeps them safe, and — at the summit — the theory of computation itself, from finite-state machines to the limits of what any computer can ever do. The topics below map the OCR/AQA-style A-level Computer Science specification; work each group in order.