The corpus luteum is a transient endocrine gland that is specialized for the production of progesterone and that plays a critical role in the establishment and maintenance of pregnancy. The life span of the corpus luteum varies between species and, within a species, can be dramatically altered by events such as mating or pregnancy. Regardless of the duration of its life span, the corpus luteum eventually enters a dynamic regression process during which it loses the capacity to produce progesterone and undergoes structural involution. 1 Table 1 Results of a cursory MEDLINE/PubMed search performed on November 2002 using the terms as listed. Rank Terms Total Number of Articles Retrieved Retrieval Period Number of Articles Retrieved for Period 2001–2002 1 "luteolysis" 1251 1967–2002 96 2 "luteal regression" 403 1966–2002 35 3 "functional luteolysis" 82 1973–2002 5 4 "structural luteolysis" 72 1969–2002 14 5 "luteal involution" 41 1978–2002 8 6 "functional luteal regression" 10 1984–2002 0 7 "structural luteal regression" 5 1988–2002 1 Terminology: a mark of scientific progress  1 2 3 4 1 et al.  5 1 6 7 et al.  8 9 et al 10 1 11 13 3 14 17 Conclusions luteal regression