In the past decade, the European Commission revised the evaluation criteria for price-based exclusionary abuses. In doing so it reformed the equally efficient competitor test and further stressed its importance. The author analyses the competitor test procedure and its implications with regard to legal certainty. First, the differences between this test and other evaluation criteria are presented, and the test's role for the Commission's more economic approach is discussed. Subsequently, the fundamental test application problems of how to choose a suitable efficiency standard, cost standards, and an adequate analysis period are addressed. Finally, the application of the competitor test on predatory pricing, margin squeezes, conditional rebates, mixed bundling, and price discrimination are outlined and evaluated. Thereby, the author focuses on the question of whether or not legal certainty can be guaranteed in the different test procedures.