Health and illness affect every interrelated aspect of all our lives. Many causal influences on health are unseen by the naked eye (viruses) but can have immense global effects, which are very varied and partly unpredictable. Health is a process, daily affected by contexts, policies, behaviours and beliefs. Health policies are practical and ethical as well as scientific, and often fail. These views run through the book on how critical realism helps us to understand them more fully. Contradictions between mainstream health research paradigms confuse and undermine the research, and theories for more valid and convincing research are considered. The chapter includes sections on the readership and aims of the book and a summary of all the chapters.