The demographic transition refers to the dramatic upward shift in fertility (i.e., birth) rates, and corresponding changes in family characteristics, that followed the Second World War, and the equally dramatic decline in those rates beginning in the 1960s and 1970s. Birth rates in Canada and Ontario after the Second World War rose to levels not seen since the 1920s. These rates, which peaked in 1961-63 at 25 out of every 1,000 population, were among the highest in the western world. The subsequent decline (the baby bust) was equally sharp, falling to a level below 15/1000 in the 1980s. The much-anticipated echo-boom (children born to the initial baby-boom population) was relatively muted. By 2001, fertility rates had fallen below 11/1000.

Although the "transition" was essentially complete by the 1980s, the implications will be with us for another half century. The primary outcome has been the very uneven size of age cohorts, from the small pre-1945 cohort to the very large baby-boom cohort to the much smaller baby-bust and echo generations. With declining fertility rates, in turn, the population has been aging rapidly. The proportion of the population over 65 years is likely to double in the next two decades, from 12 to 25% by 2021; the proportion under 19 years will decline sharply to 2021 and then (probably) stabilize.