In his Parts of Animals, Aristotle distinguishes three modes of the necessary.However, it is not clear just what these three modes are.Nor is it clear how this passage fits with other texts where Aristotle distinguishes modalities in different ways.Here I present and explain Aristotle’s three modes of necessity, and claim that they are the only three recognized by Aristotle.I then explain how this passage agrees with other passages where Aristotle mentions formal and structural features of the modalities.I end by showing how having three modes of necessity does not make ‘necessary’ ambiguous.Rather, I claim, Aristotle has a single, central notion of necessity and hence a unified theory of modality