Effective antitrust enforcement is of crucial importance for countries with a market-based health care system in which hospitals are expected to compete. Assessing hospital market power - a central issue to competition policy - is, however, complicated because the presence of third party payers and the general unobservability of prices make it difficult to apply the standard methods of market definition. Alternative, less formal methods historically employed in the hospital industry have proven inaccurate; these methods were even called inapplicable in a recent US court decision. In this paper, we discuss the strengths and weaknesses of several new approaches to defining hospital markets that are suggested in the recent economic literature. In particular, we discuss the applicability of the time-elasticity approach, competitor-share approach, and option-demand approach to the recently partly deregulated Dutch hospital market. We conclude that the appropriate approach depends crucially on how health insurers contract with hospitals and how patients select their hospital.