Central to the debate over school choice has been the question of how public schools respond to market-based competition. Many choice advocates suggest that competition can spur public schools to become more effective and efficient, but the evidence regarding the effect of competition from charters is comparably sparse and mixed. This article contributes to that literature by examining the relationship between competition, in the form of charter schools, and public school district effectiveness and technical efficiency. The investigation uses stochastic frontier analysis to analyze 6 years of statewide data in Colorado. Results indicate that charter competition appears to spur greater achievement in public schools but not greater efficiency. (Contains 8 tables.)