 Development is a central biological process, and ideas about its nature have been influential in biological thought. This entry surveys the history of these ideas through the lens of “epigenesis vs. preformation”. Epigenesis is, roughly, the thesis that every developing entity starts from material that is unformed, with form emerging gradually, over time, in the process of development. Preformation, in contrast, is the thesis that development begins with the entity in some way already preformed, or predelineated, or predetermined. The question “epigenesis or preformation?” is in part metaphysical: what is it that exists—form or also the unformed that becomes the formed? And it is partly epistemological: how do we know—through observation or inference? Debate on these entangled questions has persisted since ancient times, and today plays out as genetic determinists appeal to the already “formed” through genetic inheritance, while others insist on the efficacy of environmental plasticity. This entry surveys the main theories of development that engage this debate, from Aristotle on generation, to recent systems-theoretic and stem cell-based views. Of course, “epigenesis vs. preformation” is not the only longstanding theoretical opposition that bears on development. But this framing is an inclusive way to capture patterns of transformation and constancy in debates about biological development. Nature or nurture, epigenesis or preformation, genetic determinism or developmental free will, or is some version of a middle ground possible? The terms of this perennial discussion, and the underlying assumptions, continue to shape debates about when life begins and have profound bioethical and policy implications.