This study analyzes the timing, magnitude, and volume of the mid-twentieth century baby boom in European and non-European Western countries. The baby boom is found to have been especially strong in the non-European countries, fairly strong in some European countries, and quite weak in others. While the boom has often been linked with postwar economic growth and the recuperation of births postponed during the Depression era, we argue that this is only a limited part of the story. In most cases the recovery of the birth rate started well before the end of World War II, a fact not accounted for by existing theories. We investigate the roles played by the recovery of period as well as cohort fertility, the underlying marriage boom, and the recovery of marital fertility. We identify major puzzles for future research, including the reasons for strongly declining ages at marriage and the role played by contraceptive failure in the rise of high-parity births.