Aristotle claimed that each science is grounded in principles that are proper to it. He also recognized, however, the existence of principles that are common to many sciences, that he called axioms. The sources of Aristotle's theory of axioms seem to have been both philosophical and mathematical, and the outcomes of the theory are some very refined views on the analogy between different scientific domains, and possibly a theory of nonsyllogistic inferences. I sketch here Aristotle's theory of axioms, its motivations, its developments, and its applications to the theory of science. Finally, I offer a tentative interpretation of axioms that may shed some light on several open interpretative questions in Aristotle's epistemology.