Aristotle contrasts standard animal generation with ‘spontaneous generation’, which happens when some material putrefies and gives rise to a new organism. This paper addresses two interrelated puzzles about spontaneous generation. First, is it of the same ‘fundamental kind’ of causal process as standard generation? Second, is it ‘spontaneous’, as understood in Physics 2.4–6: rare, accidentally caused, and among things that are for the sake of something? I argue that both puzzles turn on the same questions about the process types involved. (...) I show that the type putrefaction plays a more important role in spontaneous generation than has been recognized so far. Because putrefaction does not play this role in standard generation, the two processes are of different ‘fundamental’ kinds. Moreover, spontaneous generation happens rarely in that it is rare for processes of putrefaction to happen in such a way that they can also be described in terms of concoction. (shrink)