The paper develops a simple theory of segmentation and fee-setting in certification markets. The basis for the theory is that certifiers offer differentiated tests; for a given object it is more difficult to pass the test of certifier i than the test of certifier j. Given the test standards, certifiers compete for customers via their fee-setting. Segmentation occurs in equilibrium: sellers with low unobservable quality self-select to an easy test and sellers with high unobservable quality self-select to a hard test. Moreover, sellers choosing an easy test pay a lower (endogenous) certification fee than sellers choosing a hard test. These results are consistent with evidence from the market for auditors and other markets, not readily explained by existing theories.