This study uses the 1977-1978 Kenya Fertility Survey and the 1989 Kenya Demographic and Health Survey to examine trends and determinants of contraceptive use in Kenya. A substantial increase in contraceptive use occurred in Kenya over the decade. Yet although the increase was shared by all groups in the country, the amount of change differed greatly. A regression decomposition procedure revealed that shifts in population composition--that is, an increase in better educated women and in the proportion of women who want to cease childbearing--were the primary sources of aggregate change in contraceptive use.