This chapter discusses the literature on ad hoc theory adjustment both within economics and within the Popperian philosophical tradition. It will be shown that there are two fundamentally different concepts of ad hocness within the Popperian tradition, one associated with Popper himself and one with Lakatos, and that these two different concepts are mirrored in the way the term is used by economic methodologists and economic theorists, respectively. In Section I, Popper's use of the term “ad hoc” and its fundamental importance to the Popperian program will be discussed. In Section II, Lakatos's three different notions of ad hocness will be reduced to two (one the same as Popper's and one uniquely Lakatosian), and the importance of each of these to the methodology of scientific research programs will be examined. These first two sections, although simply surveys, are necessary because this particular aspect of both the Popperian and Lakatosian philosophy of science has been badly neglected in the recent literature on economic methodology. Section III discusses the use of “ad hoc” by both economic methodologists and economic theorists, and compares these uses with those in the philosophical literature. In the conclusion (Section IV) the methodological implications of the discussion in the first three sections will be examined. It will be argued that proper emphasis on the different notions of ad hocness, and the different groups who tend to emphasize each use, have significant implications for economic methodology, particularly Lakatosian economic methodology.