Parasite fitness is tightly controlled by host ecology. The timing of seasonal host activities, or host phenology, likely impacts parasite fitness by determining transmission between infected and uninfected hosts. Changes in host phenology are also expected to drive parasite adaptation in many disease systems, yet the quantitative and qualitative impact of phenology remains under-explored. The overarching goal of this dissertation is to develop theory on how host phenology impacts both parasite transmission and parasite evolution. A novel modelling framework was developed to study how tick life-stage phenology impacts the transmission of Borrelia burgdorferi in the Lyme disease system. This study reveals that slightly asynchronous tick developmental-stage phenology results in high B. burgdorferi fitness compared to synchronous tick activity. Surprisingly, B. burgdorferi is eradicated as asynchrony increases further due to a feedback from mouse population dynamics. A model extension reveals that intermediate parasite virulence is adaptive in the absence of the classic virulence-transmission trade-off for obligate-killer, monocyclic parasites that complete one generation per season. These results suggest that host phenology could drive virulence evolution in some natural systems. A second model extension demonstrates that host phenology can drive multi-season epidemic cycles due to a feedback between host demography and parasite fitness. Short seasons and synchronous host emergence support parasite densities high enough to drive cycling dynamics as parasites adapt. Further, cycling dynamics generate an evolutionary feedback that slows parasite adaptation by preventing adaptive parasite mutants from invading when host densities have been driven down by high parasite densities. A final model extension reveals that host phenology creates multiple evolutionary stable strategies separated by evolutionary repellors for obligate-killer parasites with no constraints on the number of generations they complete per season. Certain environments support both monocyclic and polycyclic parasites, providing clues on the evolutionary origins of both strategies in nature. Overall, this dissertation contributes theory on the impact of host seasonality for parasite fitness and adaptation, providing a framework to study how species respond to seasonal change and predict how disease systems could respond to the impending climate crisis.