Over the past two decades, photocatalytic synthesis has emerged as a major theme of research within synthetic organic chemistry. This recent surge of interest has been motivated by the uniquely facile access offered by photocatalysis to high-energy reactive intermediates such as open-shell radicals and electronically excited states. The high reactivity of these intermediates enables the development of transformative new reactions, but their characteristically short lifetimes present a challenge for studying the mechanisms of these reactions. Transient absorption (TA) spectroscopy offers a powerful tool that can provide insight into the identity of photocatalytically generated intermediates, the kinetics of their elementary steps, and the participation of competitive unproductive processes. While this technique has only recently been applied to the interrogation of complex synthetic reactions, the extension of this tool into the investigation of photocatalytic mechanisms has provided insights that are not readily available using any other technique. This chapter introduces the principles of transient absorption spectroscopy and presents four case studies that highlight its use in the study of photocatalytic reactions. These studies demonstrate that TA spectroscopy provides insights that are complementary to those available using the conventional tools of physical organic chemistry.