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. Induction of matrix metalloproteinases as a consequence of activator protein (AP)-1 and nuclear factor (NF)-KB activation as well as mutations of mitochondrial DNA have been identified recently. This has increased our understanding of photoaging significantly and has led to new prophylactic and therapeutic strategies aimed at the prevention and repair of the detrimental effects of chronic sun-exposure on the skin.