Migration patterns in Kenya in general differentially affect fertility levels, patterns and behaviour. For economically dynamic areas, out-migration and the associated spousal separation and differential sex ratios seem to be associated with falling fertility, though migration is not the only or even the principal factor involved. In lagging and/or peripheral areas, by contrast, the demographic effect of migration seems to promote high fertility by undermining some critical factors of fertility determinants and behaviour. Examples from Africa in general and Kenya in particular are invoked to examine and explain this important association between migration and fertility. Furthermore, the paper incorporates the geography of economic and social development to help understand the relationship between migration and fertility.