Most habitats exhibit seasonal changes in environmental conditions. These seasonal patterns can vary among years, which can alter the timing of the seasonal life-history events, or phenologies, of species (e.g., emergence from dormancy, migration, reproduction). Species, and individuals within species, oftentimes differ in their phenological responses to year-specific conditions, which could alter the stage at which species interact. We do not have a good understanding of how these shifts in the timing of species interactions affect the outcome of those interactions, but determining the consequences is critical for understanding the dynamics of natural communities. Here, I use a series of experiments with communities of pond-dwelling insects and amphibians to determine how shifts in phenological timing affect intra- and interspecific interactions and whether these effects scale up to alter demographic rates, community structure, and ecosystem functioning. Specifically, I manipulated the mean hatching time (i.e., time of arrival to the habitat) for one species relative to another and/or the amount of variation in arrival time by individuals of a species around a mean date (i.e., degree of synchrony). First, I manipulated differences in mean arrival time for two species that interact as intraguild predators and found that the changes in relative size mediated by shifts in arrival time had strong effects on interactions. Specifically when arrival differences were small, the two species coexisted in similar abundances, but when arrival differences were large, the early arriver excluded the late arriver through predation. Second, I manipulated mean arrival time of one species relative to a predator and a competitor to determine how phenological shifts affected predator-prey and competitive interactions. I found that shifts in the arrival time of a single species within a community can affect the outcome of both interaction types strongly enough to alter community structure, and these changes to community structure scaled up to affect one of three ecosystem-level processes measured. Third, I manipulated variation in the synchrony of arrival, as well as initial density, of a species to determine the consequences for intraspecific competition. I found that variation in synchrony altered several demographic rates of the species, and these effects were density dependent. Finally, I used a factorial manipulation of the mean and synchrony of arrival by a prey species in the presence of a predator to determine how this variation affected predator-prey interactions. I found that the effects of variation in these two aspects of phenology on prey survival were additive, with survival declining with later arrival and lower arrival synchrony. Taken together, these results clearly demonstrate that shifts in phenological timing can have strong effects on intra- and interspecific interactions. These effects of phenological shifts on species interactions frequently scaled up to alter the structure of communities, and were even capable of affecting ecosystem-level processes. This work represents an important and novel contribution to our understanding of the dynamics of seasonal communities and will also be useful in understanding how climate change will alter these dynamics.