The primary objective of this thesis is to account for the fertility decline among Abagusii women in South-western Kenya. The motivation behind the study was based on the fact that the fertility trends associated with this group of people had not been studied before. This is surprising because the Abagusii recorded one of the world's highest total fertility rates of 10.4 in 1979 (Omosa 1994; Osiemo 1986; Oucho 1990), but has also experienced significant fertility decline. The total fertility rate in 2004 stood at 4.7, (Anyara 2009). The analytical framework that is adopted is Stover's (1998) adaptation of the Bongaarts et. al. (1984) Proximate Determinants Model. These determinants such as contraceptive use, post-partum insusceptibility, sexual activity, abortion and sterility represent behavioural and biological variables that affect fertility directly. The manner in which they are employed involves using the index which indicates the extent to which each proximate determinant reduces the fertility of the group in question.