There are thousands to millions of follicles in the mammalian ovary, and the majority (99.9%) are eliminated by a process known as atresia. This phenomenon occurs in any stage of follicular development, through an apoptotic manner or the degenerative process of necrosis. Thus, a better understanding of the mechanisms involved in atresia is necessary to avoid the great follicular loss that occurs in vivo and to maximize female reproductive potential. The present review focuses on aspects related to follicular population and atresia, mechanisms of atresia (apoptosis or the degenerative process of necrosis), techniques used to analyze atresia in ovarian follicles, and the occurrence of the atretic process during different follicular stages.