Our lab has been investigating fluids with light-tunable rheological properties (i.e., “photorheological” or PR fluids). We recently reported a class of aqueous PR fluids that exhibit a 10,000-fold drop in viscosity upon UV irradiation (JACS 2007, 129 1553). In this system, light caused a well-defined drop in viscosity due to a transition from long to short micelles. Here, we report the design of non-aqueous fluids that can show either an increase or a decrease in viscosity upon irradiation with light. These fluids are based on simple, inexpensive molecules rather than complex photosurfactants. The principle behind the light responsiveness of these fluids lies in controlling the ability of molecules to assemble into cylindrical chains or fibrils. This is coupled with the fact that certain molecules undergo changes in their geometry in response to light (photo-isomerizations). We will describe ways to control both the magnitude of the photorheological effect as well as its timescale in these fluids. Results from rheological, spectroscopic, analytical and scattering techniques will be presented.