It is widely agreed that ad hoc hypotheses are methodologically undesirable; they decrease the degree of a confirmation of a theory for which they are invoked. But how are ad hoc hypotheses to be understood? This chapter offers an epistemological analysis of the notion of ad hocness. Although seemingly straightforward, a descriptively adequate account of ad hocness, which goes beyond stating the motivation for introducing ad hoc hypotheses, is not easy to come by. Accounts that spell out ad hocness as the lack of testability, as the lack of independent support, as the lack of unifiedness, or as mere subjective projections are all unsatisfactory. Instead, this chapter proposes that ad hocness has to do with the lack of coherence between the hypothesis in question and (i) the theory which the hypothesis is supposed to save from refutation or (ii) the background theories at the time. This ‘coherentist conception of ad hocness’ offers another argument for realism, namely the ‘argument from coherence’.