These notes offer a comprehensive and simplified introduction to Computer Science for secondary school students, beginners in ICT, and KCSE candidates. The content is carefully structured to help learners understand core Computer Science concepts using clear explanations, practical examples, diagrams, tables, and revision summaries. The notes cover essential topics such as the history and evolution of computers, generations and types of computers, computer hardware and software, input and output devices, storage devices, computer memory, operating systems, data and information, internet basics, cyber security, ethics in computing, and emerging technologies. Designed with students in mind, the notes use simple and easy-to-understand language to make learning enjoyable and effective. They also include revision questions, key points, exam-focused explanations, and practical illustrations to support classroom learning and KCSE preparation. These notes are ideal for Form 1–4 students, teachers, revision groups, and anyone interested in building a strong foundation in Computer Science and digital literacy. Prepared professionally under GManTech Solutions LTD.