It is not uncommon for philosophers to maintain that one is obliged to believe nothing beyond the observable consequences of a successful scientific theory. This doctrine is variously known as instrumentalism, fictionalism, constructive empiricism, theoretical skepticism and the philosophy of “as if”. The purpose of the present paper is to subject such forms of scientific antirealism to a two-pronged critique. In the first place it is argued that there is no genuine difference between believing a theory and being disposed to use it to make predictions, design experiments, and so on; so traditional instrumentalism is incoherent. In the second place, a retrenched position is considered in which theoretical belief would be tolerated but said to be justified on merely pragmatic, and not epistemic, grounds. In criticizing this point of view it is shown that the onus of proof rests on anyone who maintains it; furthermore, the only possible rationale for it (which is based on underdetermination of theory by data) is described, and various deficiencies in this argument are exposed.