Spontaneous generation, the idea that some animals spring into life from nonliving matter (maggots from rotting meat, eels from mud) was a remarkably tenacious idea in the history of biology, not completely disappearing until almost the dawn of the twentieth century. Focusing mainly on the period from antiquity through the Renaissance, this paper argues that there were very good reasons for believing in spontaneous generation, but that at the same time, the phenomenon posed major theoretical problems that had to be overcome. How exactly does nonliving matter turn into living matter? What processes must it undergo to change from dead, frothy mud into a school of small fish? How, in short, does material--inert earth, air, water, fire--become life?