Analogical models can be powerful aids to reasoning, as when light is explained in terms of water waves; or they can be misleading, as when chemical processes are thought of in terms of life processes such as putrefaction. This paper proposes a structural characterization of good science analogy using a theoretical approach in which complex metaphors and analogies are treated as structure-mappings between domains. To delineate good from poor science analogy, a series of comparisons is made. First, methaphor and analogy are contrasted with literal similarity; then, explanatory-predictive analogy is contrasted with expressive metaphor; finally, within science, good explanatory analogy is contrasted with poor explanatory analogy. Analogies of historical importance are analyzed and empirical findings are discussed. (Author/CS)