Many studies on birds have shown advancing dates of egg-laying in response to climate change. This chapter reviews the latest knowledge about the roles of photoperiod, food abundance, body condition, and hormones in regulating the timing of egg-laying. A variety of responses and predictors have been discovered in recent studies, some of which may be related to whether a species relies on daily food intake (‘income’) or stored resources (‘capital’) for breeding. The literature on advancing laying dates shows that the rate is dependent on the number of broods per season, habitat, and trophic level. Mismatches between timing of breeding and peaks in food supply are often mentioned as a potential threat to populations. However, to date, there is no association between changes in laying date and population trends, which suggests that the effects of climate change on bird populations may be driven by other factors.