Proliferation is an important characteristic of organisms. Both unicellular and multicellular organisms reproduce, resulting in new organisms similar to themselves. They can produce any number of organisms. Unicellular organisms proliferate by cell division. Multicellular organisms not only show multiplication of individual cells but also undergo morphogenesis during which a multicellular body are generated from a zygotic cell. The organism acquires its structure through differentiation and growth of tissues and organs. Environmental influences are very important for proliferation. This chapter provides a basic description of these processes.