Structure mapping theory is a way of characterizing analogies and certain classes of metaphors. The central claim of this paper is that all analogies and many metaphors are fundamentally devices for mapping relational structures from one domain to another. This theory differs from other theories in postulating that the interpretation rules for analogies and relational metaphors are based on predicate structure rather than on feature salience or mental distance. Two experiments tested the interpretive predictions of the structure mapping theory as well as those derivable from A. Ortony's salience imbalance theory of metaphor. Subjects, college students, were asked to interpret metaphors and rate their aptness and "metaphoricity," after first writing out descriptions of all the object terms used in the metaphors. Results supported the structure mapping theory of metaphor. Specifically, the experiments revealed that: (1) metaphor interpretations were more relational than the object descriptions on which they were based, suggesting that the subjects found metaphors more apt to the extent that they could find a relational system to map from base to target object; and (2) adult subjects produced structural interpretations of metaphorical comparisons when possible and judged the aptness of the metaphors according to their relationality, suggesting that metaphoric development can be described as the gradual development of relational focus. (A seven-page reference list is included.) (FL)