Christianity—secret adherence to Christian religious practices by people who outwardly professed Islam—is known to have occurred in several parts of the Ottoman Empire; this essay concerns the crypto-Christians of Kosovo, who were very unusual in adhering to Roman Catholicism. Distinctions are made here between crypto-Christianity and a range of other practices or circumstances that have been partly confused with it in previous accounts: the fact of close social coexistence between Muslims and Christians; the existence of religious syncretism, which allowed the borrowing and sharing of some ritual practices; and the principle of ‘theological equivalentism’ (the claim, made by some Muslims, that each person could be saved in his or her own faith). These things were not the same as crypto-Christianity, but they involved different kinds of religious ‘amphibianism’, creating conditions in which crypto-Christianity could survive more easily. The story of Catholic crypto-Christianity in Kosovo and northern Albania begins with reports from Catholic priests in the seventeenth century. Contributory factors seem to have been the economic incentive for men to convert to Islam to escape the taxes on Christians, and the fact that women (who were not tax-payers) could remain Christian, as Christian wives were permitted under Islamic law. This essay then traces the history of the crypto-Catholics of Kosovo, who survived, despite the strong official disapproval of the Church, into the nineteenth and twentieth centuries.