My aim in this paper is to investigate the epistemic functions metaphors might perform. According to a traditionally influential idea metaphors have, at best, ornamental value; they are poetic or rhetorical devices, used to please or even sway people. Current research in philosophy, linguistics and psychology shows the need for a refined picture of what purposes metaphors might serve. Expressions are commonly used metaphorically in order to conceptualize abstract and mental phenomena. The expressions thereby employed are often taken from the realm of sense experience; we feel blue, or complain about someone being cold, and so on. Yet even in the natural sciences metaphors have added epistemic value. They direct our attention to phenomena that we did not hitherto notice, make us think thoughts that we did not think before, etc. More specifically, I will claim that metaphors have heuristic, exploratory and explanatory value. Nonetheless, some metaphors are more successful than others. I will close by advancing various criteria for metaphorical success and failure.