Nature uses light in an organized way in order to accomplish complex reactions such as those of photosynthesis. However, part of these reactions can become uncontrolled, generating reactive species that damage cells and tissues by a process called photosensitization in which chromophores absorb light and transfer its energy to neighboring molecules. Photosensitization reactions affect the yield of photosynthesis as well as are the main causes for skin damage, which is caused by UVA and visible light. We will explore the mechanisms by which the natural chromophors (chlorophils, flavins, melanins, lypofuscin) absorb light and act as photosensitizers, damaging intracellular organelles and tissues. These reactions provide a mechanistic explanation for skin photoaging activated by visible light. Man has learned to use these same reactions to actively induce cell death of diseased tissues and consequently to treat several diseases with light, in a process called Photodynamic Therapy (PDT). In PDT synthetic photosensitizers (molecules and/or nanoparticles) are used to efficiently induce photosensitized reactions. We aim to use the photosensitization reactions to specifically trigger controlled mechanisms of cell death. We will show that by causing small damages in mitochondria, apoptotic cell death is efficiently induced, while if there is parallel damage in mitochondia and in the lysosome, autophagic cell death is induced. With this knowledge we hope to develop photosensitizers that have more specific biological targets aiming to improve the efficiency of PDT protocols.