 Developmental biology has increasingly become an area of exploration for  philosophy of biology  due to the potential relevance of development for understanding evolution (Love 2015;  Section 5),  the theme of reductionism in biology and explanations from molecular genetics (Robert 2004; Rosenberg 2006;  Section 3),  and via increased attention to the details of particular research programs, such as stem cell biology (Fagan 2013; Laplane 2016). However, it should not be forgotten that ontogeny was on the radar of philosophical scholars in the 20th century, as seen in Ernest Nagel’s treatment of hierarchical organization and reduction in the development of living systems (Nagel 1961: 432ff). For contemporary philosophy of science, developmental biology displays a rich array of material and conceptual practices that can be analyzed to better understand the scientific reasoning exhibited in experimental life science (see the entry on  experiment in biology).  After a brief review of some central phenomena of ontogeny, this entry explores four domains that represent some of the import and promise of conceptual reflection on the epistemology of developmental biology.