Aimed at advanced undergraduates, this self-contained textbook covers the key ideas of special and general relativity together with their applications. The textbook introduces students to basic geometric concepts, such as metrics, connections and curvature, before examining general relativity in more detail. It shows the observational evidence supporting the theory, and the description general relativity provides of black holes and cosmological space-times. The textbook is in full colour, with numerous worked examples and exercises with solutions. Key points and equations are highlighted for easy identification, and each chapter ends with a summary list of important concepts and results. This textbook provides the essential background for an up-to-date discussion of modern observational cosmology. Each chapter builds on the previous one as concepts are developed, making it ideal for self-study.