The threespine stickleback is a small fish of temperate coastal and fresh waters. It exhibits extraordinary phenotypic diversity throughout its range, and is remarkably amenable to observation in the field and manipulation in the laboratory. This volume incorporates reviews from active workers who use the threespine stickleback to address a broad variety of evolutionary issues, ranging from optimal foraging to armour, from speciation to the endocrine basis for correlated behavioural characters. It highlights the fundamental insights into evolutionary processes that have emerged from these studies.