Climate change and other anthropogenic drivers alter seasonal regimes across freshwater, terrestrial, and marine biomes. Seasonal patterns affect ecological and evolutionary processes at different ecological levels through changes to gene frequencies, species traits, population dynamics, species interactions, and different facets of biodiversity. We synthesize the mechanisms that determine biological responses to seasonality, to demonstrate how their interconnections can propagate impacts of altered seasonal patterns and complicate predictions. Given the potential for nonlinearities and the propagation of impacts across levels of ecological complexity, we advocate the use of mechanistic approaches that acknowledge species-specific responses to the environment and potential seasonal adaptations.