Decades of theoretical and empirical research have contributed numerous ways to measure competition and to compare the competitive impact of alternate regulatory policies and market environments. Several of the most convenient measures, unfortunately, are beset by very serious problems, while none are completely ideal. Faced with an ongoing and undiminished need to assess competition and market power nonetheless, we would advocate a focus on the scant handful of “least objectionable†measures. Among these, the Lerner index and the Rothschild–Bresnahan conduct index together provide complementary, well-established, easily understood measures that relate to policy-relevant aspects of market power according to formal underlying theoretical models of firms and industries. The latter approach is slightly more demanding with regard to data and estimation techniques, requiring nonlinear systems estimation except in a correlation version under additional assumptions; one tradeoff is that the correlation version yields only qualitative (rather than quantitative) conclusions about market power.