Background/Question/Methods Shifts in phenology, such as flowering and bird migrations, are among the best-documented biological responses to climate change. Species vary, however, in the magnitude of their responses. For example, some plants are flowering much earlier than in the past, some slightly earlier, while others are not changing at all or are even flowering later. Similar variation appears to be present at all trophic levels and has the potential to greatly alter time-sensitive ecological interactions, such as those between plants and pollinators, plants and herbivores, and predators and prey. We used long-term phenological records dating back to 1852 from Massachusetts, Colorado, and Japan to examine factors that explain some of this variation among species and to predict possible ecological consequences. Results/Conclusions Factors that explain variation in phenological responses to climate change include methodological, climatological, and ecological factors, and interactions among them. In particular, different species may respond to different climate variables, such as temperature, snowmelt, precipitation, or wind. Ecological factors, such as time of phenology (e.g., early season vs. late season) and habitat explain additional variation. For example, on average the phenologies of early-season plants and migratory birds are changing more quickly than are those of late-season plants and birds. Moreover, some of these patterns lead to predictable ecological consequences. For example, in many cases gaps may be forming between early- and late-flowering plants, creating periods of low floral resources for some pollinators.