Photochemistry plays a central role in synthetic polymer research. Aromatic ketones, examples of which include benzophenone, thioxanthone, camphorquinone, among others, are renowned for their excellent optical characteristics and have been extensively utilized for photochemical induction of polymerization processes. Of particular interest is thioxanthone due to its adaptability for bearing different functionalities and its applications in various modes of photopolymerization, in which it accomplishes photoinitiation in conjunction with other co-initiator compounds; a behavior that is referred to as bi-molecular photoinitiation. In this paper, we review the photochemistry of thioxanthone-based systems and their use in different modes of photoinitiated polymerizations. Citing examples from the literature, the development of various photoinitiating systems based on thioxanthones along with an understanding of their mechanistic behavior has been elucidated previously.