 The description of scientific realism as a positive epistemic attitude toward theories, including parts putatively concerning the unobservable, is a kind of shorthand for more precise commitments (Kukla 1998: ch. 1; Niiniluoto 1999: ch. 1; Psillos 1999: Introduction; Chakravartty 2007a: ch. 1). Traditionally, realism more generally is associated with any position that endorses belief in the reality of something. Thus, one might be a realist about one’s perceptions of tables and chairs (sense datum realism), or about tables and chairs themselves (external world realism), or about mathematical entities such as numbers and sets (mathematical realism), and so on. Scientific realism is a realism about whatever is described by our best scientific theories—from this point on, “realism” here denotes scientific realism. But what, more precisely, is that? In order to be clear about what realism in the context of the sciences amounts to, and to differentiate it from some important antirealist alternatives, it is useful to understand it in terms of three dimensions: a metaphysical (or ontological) dimension; a semantic dimension; and an epistemological dimension.