In 1875 Francis Galton was the first to study twins as a test of the relative strength of heredity and environment. This paper examines Galton's work on twins, using his surviving working papers. It shows that his enquiry was larger and more systematic than previously realized. Galton issued several hundred questionnaires to parents of twins, with the aim of establishing how far the similarities and differences between twins were affected by their life experiences. The paper also discusses Galton's study in relation to his understanding of the physiology of twinning and his theory of heredity. The modern concept of monozygotic twins had not yet been established, and the similarity between Galton's work and modern twin studies should not be overstated. While Galton's work was important as a pioneering study, in some respects his conclusions went beyond his evidence. The paper finally examines whether Galton's twin studies influenced his position on the links between social class, heredity and social mobility, and surveys the evidence for his views on these issues.