ion was investigated by examining extrapolation behavior in a function-learning task. During training, participants associated stimulus and response magnitudes (in the form of horizontal bar lengths) that covaried according to a linear, exponential, or quadratic function. After training, novel stimulus magnitudes were presented as tests of extrapolation and interpolation. Participants extrapolated well beyond the range of learned responses, and their responses captured the general shape of the assigned functions, with some systematic deviations. Notable individual differences were observed, particularly in the quadratic condition. The number of unique stimulus-response pairs given during training (i.e., density) was also manipulated but did not affect training or transfer performance. Two rule-learning models, an associative-learning model, and a new hybrid model with associative learning and rule-based responding (extrapolation-association model [EXAM]) were evaluated with respect to the transfer data. EXAM best approximated the overall pattern of extrapolation performance. Research on conceptual behavior has historically focused on category learning and the application of categorical knowledge. So dominant is this focus that the terms concept and category are often used interchangeably (e.g., see Bourne, 1966; Smith & Medin, 1981). It is useful to distinguish these two terms, however, because there are many types of concepts that can not be adequately characterized as categories (Busemeyer, McDaniel, & Byun, 1997; Estes, 1995). In general, concepts also pertain to causal variables (e.g., intelligence) and relationships between these variables (e.g., income is correlated with intelligence). This article investigates functions, which conceptualize the relationship between causal variables. By definition, a function maps a set of input values (called the domain of the function) into a set of output values (called the range of the function), such that each input value is assigned only one output value. In function-learning situations, the range is composed of a continuous set of response magnitudes (e.g., predict a student's GPA on the basis of IQ scores). In category learning, on the other hand, outputs consist of discrete and nominal response categories (e.g.,