Photosensitizers (PSs) are light-responsive compounds that are significant in therapies and imaging. They are at the forefront of advancements in phototheranostics. They generate reactive oxygen species (ROS) upon exposure to specific wavelengths of light. This chapter provides a brief overview of their significant role, molecular structure, and various applications in phototheranostics. The chapter also examines the mechanisms of action and their applications in imaging and therapeutics. The therapeutic use of these PSs is further explored through examples in photodynamic therapy (PDT), photothermal therapy (PTT), and combination therapies. Finally, we discuss the ongoing challenges in selecting these PSs, highlighting how innovative strategies and emerging advancements in phototheranostics could provide effective solutions.