What is development, and how should it be explained? Conceptual issues play an important role in today's developmental biology (e.g., concerning plasticity and robustness, the periodization of development, etc.) An important, related question is to determine whether developmental biology offers theories. In a volume published by OUP in 2014 and edited by Minelli and Pradeu (Towards a theory of development), prominent biologists and philosophers have examined this question. This exploration of the theoretical aspects of developmental biology has now naturally led us to a broader and perhaps even more fundamental question, that of the explanation(s) of development.