This study discusses fertility trends and variation in countries that completed the transition from high to around-replacement fertility in the 1950s to 1980s, especially in Europe, East Asia and North America, and summarizes the key relevant findings for those countries with a more recent experience of fertility decline towards replacement level. A central finding is that there is no obvious theoretical or empirical threshold around which period fertility tends to stabilize. Period fertility rates usually continue falling once the threshold of replacement fertility is crossed, often to very low levels. While cohort fertility rates frequently stabilize or change gradually, period fertility typically remains unstable. This instability also includes marked upturns and reversals in Total Fertility Rates (TFRs), as experienced in many countries in Europe in the early 2000s. The long-lasting trend towards delayed parenthood is central for understanding diverse, low and unstable post-transitional fertility patterns. In many countries in Europe this shift to a late childbearing pattern has negatively affected the TFR for more than four decades. Many emerging post-transitional countries and regions are likely to experience a similar shift over the next two to three decades, with a depression of their TFRs to very low levels.