Many bacterial photoreceptors signal via histidine kinases. The light-activated nature of these proteins provides unique experimental opportunities to study their molecular mechanisms of signal transduction. One of these opportunities is the combined application of X-ray crystallography and optical spectroscopy in protein crystals. By combining these two methods it is possible to correlate protein structure to protein function in a way that is exceedingly difficult or impossible to achieve in most other experimental systems. This chapter is divided into two parts. The first part provides a brief overview of light-regulated histidine kinases and the most important techniques for studying the structure of photocycle intermediates by crystallography. The second part of the chapter is dedicated to practical advice on how to select, mount, activate, and monitor the structural and spectroscopic responses of photoreceptor crystals. This chapter is intended for readers who want to start using these experimental tools themselves or who wish to understand enough about the techniques to critically evaluate the work of others.