The aim of this book is to provide detailed coverage of the topics in the new OCR AS and A Level Computer Science specifications H046 / H446. The book is divided into twelve sections and within each section, each chapter covers material that can comfortably be taught in one or two lessons. Material that is applicable only to the second year of the full A Level is clearly marked. Sometimes this may include an entire chapter and at other times, just a small part of a chapter. Each chapter contains exercises and questions, some new and some from past examination questions. Answers to all these are available to teachers only in a free Teacher's Pack which can be ordered from our website www.pgonline.co.uk. This book has been written to cover the topics which will be examined in the written papers at both AS and A Level. Sections 10, 11 and 12 relate principally to problem solving skills, with programming techniques covered in sufficient depth to allow students to answer questions in Component 02. Pseudocode, rather than any specific programming language, is used in the algorithms given in the text. Sample Python programs which implement many of the algorithms are included in a folder with the Teacher's Pack.