Photodynamic therapy (PDT) is a new method for the treatment of both cancer or non-cancer diseases. It combines three basic elements (a photosensitizer, light, oxygen), every single one with no toxicity or biological activity. The photosensitizer (PS) is activated by light and it gets excited. It transfers absorbed energy from this excited form to oxygen forming reactive oxygen species (ROS), mainly singlet oxygen. Then ROS attack surrounding biomolecules, inhibit their biological function leading in the end to a cell death. The complete destruction of the tumour is provided not only by a direct effect on the cells, but also a vasculature shutdown (leading to nourishment and oxygen depletion) and an activation of the immunity response play an important role in this process. The present paper proposes to introduce the readers to the history and basic principles of PDT from the point of view of either the physical mechanism of PS activation or biological effects on the target tissue (on molecular, subcellular, cellular, and tissue levels).

        Key words: photodynamic therapy – singlet oxygen – photosensitizer