This textbook provides comprehensive yet concise coverage of all the topics covered in Unit A451: Computer Systems and Programming of the OCR GCSE Computing Specification J275, written and presented in a way that is accessible to teenagers. It will be invaluable both as a course text and as a revision guide for students nearing the end of their course. It is divided into seven chapters corresponding to the seven sections of the specification, each ending with a "Glossary of terms" and exam questions from past OCR GCSE papers.