In this paper the development of fertility and mortality in Finland, and their interrelations with each other and with economic factors is discussed. An analysis by individual years shows that rises and falls in mortality and fertility rates did not always coincide with poor and good harvests. Fertility in Finland decreased slightly at the turn of the eighteenth and nineteenth centuries, but fell sharply over the period 1876–1925. This fall corresponded closely to changes in the death rate, especially for infants, and appears to justify the conclusion that the changes were connected. These population shifts have been called the first and second demographic transitions, of which the latter was the more dramatic. Factors tending to reduce mortality among infants and in other age groups during the second demographic transition are obvious; those underlying the first demographic transition are less clear. In this connection, the importance of breast feeding and campaigns designed to favour the practice are stressed. These helped to reduce infant mortality and were one of the main reasons for the first demographic transition. Finnish material also suggests that some kind of family planning existed during the pre-industrial period; it is only by making this assumption that the various figures can be made compatible.