Lakes are the lentic water bodies surrounded by land and are more voluminous than pond which is entirely isolated from oceanic ecosystem. It may be natural or artificial and can be classified based on origin, trophic levels, mixing cycle of water and nature of inflow-outflow. Compared to other ecosystem, lakes are relatively discrete in nature, hence the inter-relationship between functional aspect of physical, chemical and biological processes in them is quite clear and therefore one can study, understand and manage it in effective way. This chapter highlights the formation and variety of lakes.