We present an interpretation of Land’s Retinex theory in terms of a Partial Differential Equation (PDE) that we show to be consistent with the original formulation. The proposed model can be extended into a variational formulation (i.e. a suitable modification of the PDE is shown to minimize a certain energy functional), maintaining all the advantages of Retinex but overcoming its intrinsic limitation regarding overexposed pictures. Furthermore, this variational formulation allows to show the relationship of Retinex with the ACE color enhancement algorithm (Rizzi et al. 2003) and with histogram equalization (Sapiro and Caselles 1997), and also suggests a connection with neural activity at the visual cortex.