The differences between intrinsic aging and photoaging are reviewed. The various model systems currently employed for the studies of aging and photoaging are discussed. Findings on age associated decrements in receptor/ligand mediated signaling as well as changes during cellular senescence in the expression of nuclear transcription factors are described. The role of telomere shortening and oxidative damage in the aging process is explained. At the cellular level, genetic and behavioral differences between aging and photoaging are illustrated with particular emphasis on changes in the structure and function of the tumor suppressor gene pS3. Journal of Investigative Dermatology Symposium Proceedings 3:47-51, 1998