This introductory chapter serves multiple purposes. Its primary aim is to introduce psychiatrists and other mental health professionals who are new to Darwinian thinking to some of the basic concepts and terminology of evolutionary science in order to ease their progress through the remaining chapters of this volume. Another aim is to provide a distillation and update of some significant theoretical and other developments in a variety of evolutionary disciplines relevant to psychiatry and psychology that would be of benefit to all readers, including existing evolutionists. Given the constraints of space, there will inevitably be significant omissions. We have elected to cover the basics of standard evolutionary theory, as well as some of the basic principles of evolutionary psychology and medicine. We also briefly survey some of the recent developments in the evolutionary literature on cultural evolution and related fields. We recognise that a balance needs to be struck between covering as wide an area as possible without the chapter becoming a glossary of terms. Readers unfamiliar with specialised evolutionary terms are advised to consult the glossary on the Evolutionary Psychiatry Special Interest Group at the Royal College of Psychiatrists’ website: www.epsig.org (click on ‘About us’ then ‘Resources’).