Winner of the 2022 Cheryl Frank Memorial Prize. Critical realism, as a toolkit of practical ideas, helps researchers to extend and clarify their analyses. It resolves problems arising from splits between different research approaches, builds on the strengths of different methods and overcomes their individual limitations. This original text draws on international examples of health and illness research across the life course, from small studies to large trials, to show how versatile critical realism can be in validating research and connecting it to policy and practice. To meet growing demand from students and researchers, this book is based on the course at UCL, first taught by Roy Bhaskar, the founder of critical realism.