What is a lake? At first glance, this seems like such an easy question: a lake is simply a body of water surrounded by land. But this sterile, physical definition is only a beginning, and there are so many other more interesting ways to consider the nature and meaning of lakes. The ‘Introduction’ outlines the aim of this VSI: to provide a condensed overview of scientific knowledge about lakes, their functioning as ecosystems that we are part of and depend upon, and their responses to environmental change. It considers the physical, biological, and chemical features of lakes as well as their importance in environmental sciences, engineering, and society.