All specialised cell types present in the adult human body are derived from a pool of generic cells. These generic pluripotent stem cells exist only during a short period in development and, in a process called differentiation, rapidly undergo vast changes to their gene expression resulting in the specialised morphology and function of mature somatic cells. Although remarkable progress has been made documenting and manipulating the observable changes along this developmental trajectory, a general understanding of the process of differentiation is still lacking. Here I present how the results from time-course differentiation experiments in vitro inform basic theoretical models of cell state transitions in development.