Julian Hart Lewis FRS[2] (12 August 1946 – 30 April 2014) was an English developmental biologist and researcher whose work shed light on the nature of cellular timing mechanisms and their role in animal development.

He showed that the Notch ligand (a molecule involved in cell-to-cell communication) controls the timing of nerve cell differentiation and the synchronised cycling of neighbouring cell activity.

He modelled the cellular oscillatory circuit that determines the segmentation of the developing body, and clarified the importance of delay kinetics in setting the frequency of those oscillations.

The British Society for Developmental Biology awarded him the Waddington Medal in 2003.

He died in April 2014 after living with prostate cancer for a decade.