In this International Year of Light, it is particularly appropriate to review the historical concept of what is light and the controversies surrounding the extent of the visible spectrum. Today we recognize that light possesses both a wave and particle nature. It is also clear that the limits of visibility really extend from about 310 nm in the ultraviolet (in youth) to about 1100 nm in the near-infrared, but depend very much on the radiance, that is,‘brightness’ of the light source. The spectral content of artificial lighting are undergoing very significant changes in our lifetime, and the full biological implications of the spectral content of newer lighting technologies remain to be fully explored.