The olive fruit fly is the most important pest of olives worldwide and in California, where it is a recent invader. First detected in 1998, the olive fruit fly has rapidly spread to every olive-growing region of the state. Olive fruit fly monitoring has been conducted from 2002 through 2005 at 27 sites in 15 California counties. Three to 5 overlapping olive fruit fly generations are observed annually in California, dependent upon weather and cultural conditions. Generational flights were determined through visual observation of monitoring data and confirmed by dissection of female flies to determine ovarian development and mating status. Hourly weather data collected at each location during the study was used in combination with monitoring data to determine lower and upper developmental thresholds, and develop a phenology model that predicts olive fruit fly generations. Olive development was observed at 7 locations throughout the state and correlated with fly infestation data to determine when olives are susceptible to olive fruit fly attack. The model was validated with independent data from several monitoring locations. Olive fruit fly chemical, cultural and biological control can be improved by better predictions of fly phenology enabled by the model and its linkage to olive fruit development.