A large field of investigation that includes the study of all changes associated with an organism as it progresses through the life cycle. The life cycles of all multicellular organisms exhibit many similarities. That is, as an organism progresses from one generation to the next there is a series of common processes: for example, gametogenesis, fertilization, embryogenesis, cell differentiation, tissue differentiation, organogenesis, maturation, growth, reproduction, senescence, and death.