Teratogenesis poses a real problem for all those who wish to see the natural world as a success story, and this includes the Neoplatonists. On their view even ordinary biological reproduction is governed by principles ultimately derived from intelligible Forms. Thus, the generation of terata would seem to call into question the very efficacy of these intelligible principles in the sensible world, since these would seem to be cases in which matter has gotten the upper hand over the intelligible. Although (...) the corpus of surviving Neoplatonic works offers no systematic discussion of the problems surrounding teratology, it is possible to find a number of passages which deal with the explanation of terata. In this article these passages are collected and discussed. It is argued inter alia that, far from capitulating in the face of this putative evidence against the full efficacy of intelligible causes in the world, the Neoplatonists managed to use these phenomena to the advantage of their own views. (shrink)