The use of light to drive chemical reactions is becoming increasingly popular due to the enhanced spatial and temporal control provided. Because of this, it is important to understand how these photochemical transformations occur from a mechanistic viewpoint in order to aid in the improvement of existing systems as well as in the development of new systems. The work presented in this dissertation will examine the mechanisms of several photochemical systems including photoacid generators, photoreleaseable protecting groups, and diarylnitrenium ions. Chapter 1 will begin with an introduction to organic photochemistry and describe some of the excited state reactions that will be encountered throughout this text. It will also describe laser flash photolysis, a technique critical to studying the reactive intermediates generated in photochemical reactions. Chapter 2 will describe the design and synthesis of photoacid generators that are activated via sequential two-photon absorption. The experiments conducted support a mechanism involving triplet re-excitation providing a more favorable bond scission. Chapter 3 will explore the applications of these newly developed photoacid generators, specifically for photopolymerization. It is shown that these compounds are capable of initiating both cationic and radical polymerizations depending on the intensity of visible light irradiation used. Chapter 4 will examine the 9-phenyl-9-tritylone photoreleaseable protecting group for alcohols to understand the details of its release mechanism. It is shown that the tritylone anion radical is required for alcohol photorelease. Chapters 5 and 6 will explore the behavior of diarylnitrenium ions in aqueous media. Chapter 5 will examine the reactivity of diarylnitrenium ions toward guanosine and it is shown that there is a rapid reaction to generate the C8 adduct, suggesting potential carcinogenicity. Chapter 6 will examine the reactivity of diarylnitrenium ions under acidic aqueous conditions. Under these conditions, a long-lived species is formed, and the experiments conducted indicate this species is the cation radical derived from the diarylnitrenium ion. Mechanistic analysis supports formation via a pathway separate from the nitrenium ion, suggestive of a triplet mechanism.