From the mysterious depths of Lake Vostok, Antarctica, to tropical floodplain lakes, inland seas, hydro-reservoirs and the variety of waterbodies in our local environment, lakes encompass a huge diversity of shapes, sizes, depths, colours, and even salinities. Often very large and very deep, they sustain important and unique ecosystems which can be hotspots of biodiversity, and are used by humans as sources of drinking water and food, in particular, fish. What is the origin of differences among lakes, and how does that affect the life within them? What are the seasons of a lake, and how do human actions alter lake ecosystems locally, and at a global scale? In this Very Short Introductions, Warwick Vincent outlines the essential features of lake environments and their biology, offering an up-to-date view of lake ecosystems. Vincent traces the origins of lake science (limnology) from the seminal work of François Forel on Lake Geneva at the edge of the Swiss Alps, to modern approaches such as environmental sensors, satellite observations, stable isotope analysis, and DNA-based technologies which are used to probe the microbial life support systems that lead from sunlight to fish. Drawing on varied case studies he considers the intimate relationship between humans and lakes, the value of lakes as indicators of environmental change, the impact of pollution,and our urgent need to improve the protection and management of these vitally important living resources via an integrated understanding of their ecology.