In both the United States and Canada, the environment in which physicians practice is becoming increasingly constrained. The understanding of how physicians respond to economic incentives is still relatively crude. In this article, the authors use physician-specific data for physicians in four specialties in the province of Ontario, Canada, to examine utilization trends during the 1983-1990 period and relate these utilization changes to physician, practice, and market-area characteristics. The analysis of trends in utilization reveals marked variation across specialties in the degree of change, its distribution among physicians within each specialty, and the way in which utilization growth was achieved. The physician, practice, and market-area characteristics explained a substantial amount of variation in the levels of utilization across physicians and over time; they were less able to explain year-to-year changes in utilization.