Abstract: We describe an evanescent wave cavity-based optical sensor. The sensor comprises an optical cavity formed by a pair of highly reflective surfaces and including a totally internally reflecting (TIR) surface generating an evanescent wave to provide a sensing function. The TIR surface is provided with a functionalising material over at least part of said TIR surface such that said evanescent wave interacts with said material, and whereby an interaction between said functionalising material and a target to be sensed is detectable as a change in absorption of said evanescent wave. The technique is extremely sensitive and thus in a gas sensor the evanescent wave TIR surface is provided with a solvating medium, and in other configurations a partition or atrium is provided for the evanescent wave sensing region.