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Chronic sun exposure causes photoaging of human skin, a process that is characterized by clinical, histological and biochemical changes which differ from alterations in chronologically aged but sun-protected skin. Within recent years, substantial progress has been made in unraveling the underlying mechanisms of photoaging. There is evidence that mitochondrial damage, protein oxidation, telomeres, and the synthesis of matrix metalloproteinases induced by activation of activator protein ��AP��-1 and nuclear factor ��NF��-kB play a certain role in the development of photoaging. The understanding and recognition of photoaging mechanism may lead to the development of new prophylactic and therapeutic strategies for skin damage caused by chronic sun-exposure.