There are two major theories of aging: the programmatic theory states that aging is an inherent genetic process, and the stochastic theory states that aging represents random environmental damage. Processes that are associated with cellular damage and aging are the production of free radicals (a process much enhanced after ultraviolet irradiation) and an increasing number of errors during DNA replication. Cellular manifestations of intrinsic aging include decreased life span of cells, decreased responsiveness of cells to growth signals, which may reflect loss of cellular receptors to growth factors, and increased responsiveness to growth inhibitors. All these findings are more pronounced in cells derived from photodamaged skin. Molecular manifestations of intrinsic aging, studied mainly in fibroblasts, are altered membrane composition, adhesion properties, production of extracellular matrix, and activity of the enzyme catalase. Molecular changes as a result of photoaging are less well characterized; published studies mainly target differences in composition of the extracellular dermal matrix.