We review and discuss the ultimate and proximate causes of birth seasonality in Neotropical primates and the seasonal patterns shown by each genus within this group. Our review of the literature shows that most New World monkey populations studied so far show some degree of birth seasonality. Photoperiod is the most important proximate cue used by populations living at relatively high latitudes to time their reproductive events, but almost nothing is known about the proximate factors used by those near the equator. The findings are consistent with the hypothesis that food availability is the most important ultimate cause of birth seasonality. Predation seems to promote birth synchrony in some species (e.g., squirrel monkeys). Multiple regression ANCOVA was used to estimate how the degree of birth seasonality is affected by ecological and life history variables. The ANCOVA model shows that three factors affect the degree of birth seasonality: diet, latitude, and body size. Folivores (howlers) are less seasonal than frugivores and insectivores. The degree of seasonality increases with latitude and shows a humped relationship with body size, peaking at 1.66 kg body mass. This last relationship was expected since small bodied species have to pay a cost (in terms of time lost) by being seasonal on a yearly basis, and large species are buffered against fluctuations in food availability due to their large body mass. To understand which of three alternative birth strategies is followed by each species (reduce energy stress during peak lactation, wean infants during peak food availability, or store reserves during peak energy availability), we compared the location of the birth peak in relation to the peak in food-availability for those populations from which data were available. Most species conform to the typical pattern of births concentrated before the peak in food availability, allowing peak lactation (small-sized species) or weaning (capuchins) to take place before the start of the lean season. The pattern of births of the atelines is consistent with the weaning hypothesis. However, since they give birth during the lean season, this pattern is also consistent with an alternative strategy.