 The problem of theoretical terms is a recurrent theme in the philosophy of science literature (Achinstein 1965; Sneed 1979: ch. II; Tuomela 1973: ch. V; Friedman 2011). Different shades of meaning have been associated with this problem. In its most comprehensive formulation, the problem of theoretical terms is to give a proper account of the meaning and reference of theoretical terms. There are at least two kinds of expression that pose a distinct problem of theoretical terms, respectively. First, unary predicates referring to theoretical entities, such as ‘electron’, ‘neutrino’ and ‘nucleotide’. Second, non-unary theoretical predicates, such as ‘homology’ in evolutionary biology and theoretical function expressions, such as ‘force’, ‘temperature’ and ‘intensity of the electromagnetic field’ in physics. Sneed’s problem of theoretical terms, as expounded in (1979: ch. II), concerns only the latter kind of expression. We shall now start surveying problems concerning the semantics of expressions for theoretical entities and then move on to expressions for theoretical relations and functions.