This chapter reviews the various histological, ultrastructural, histochemical, and biochemical changes occurring in the atretic follicles and corpora lutea in both cartilaginous and bony fishes. It discusses the endocrine capacity of the corpora lutea, their role in gestation and other reproductive functions, and atretic follicles, their degeneration and the possible hormonal and/or environmental factors affecting/governing follicular atresia. Development of corpora lutea in the ovaries ovulation from the follicular envelope is a ubiquitous feature of the ovaries in fish, with the exception of viviparous forms with follicular gestation. Fish exhibit diverse reproductive patterns such as oviparity, ovoviviparity, and viviparity. Various interchangeable terminologies have been used in research articles on the fish ovary to designate the degenerating follicles before ovulation and the structures that evolve from the follicular wall after ovulation in the fish ovary. Terms such as "corpora atretica," "preovulatory corpora lutea," "corpus luteum preovulationis," "corpus luteum atreticum" have been used to refer to the degenerating follicles.