Cancer is a leading cause of death worldwide. Developing targeted therapies that are selectively toxic to cancer cells while sparing normal cells may lead to more effective treatment options. Photodynamic therapy (PDT) is one of these therapies; it is a promising non-invasive treatment and a good alternative to cutting surgery or chemotherapy. PDT is a two-step process that involves the administration of a photosensitive dye (photosensitizer) leading to selective uptake and retention in the tumor cells. Following this, the photosensitizer is activated by exposure to light of an appropriate wavelength. This ultimately leads to release of reactive oxygen species which eventually cause tumor cell death. The successful clinical use of PDT requires the selection of the most appropriate photosensitizer that is preferentially accumulate in tumor cells, with a satisfactory stability in aqueous solutions and having long circulation time in the body. This book provides a new insight in developing an ideal photosensitizer with a good photodynamic activity and an ability to be targeted at the desired site of action.