追求年輕的外貌是人之天性，長久以來人們為了預防皮膚衰老已付出許多努力，人體皮膚每天都會暴露於環境壓力下因此導致急性損傷和炎症，隨著時間的流逝將會導致與過早老化相關的皮膚形態和視覺外觀變化。皮膚衰老是由內在和外在因素所共同引起的生物過程，後者主要是長期暴露於紫外線 (ultraviolet, UV) 下所導致，UVA 是引起皮膚光老化的主要原因。許多研究顯示視黃酸 (retinoic acid, RA) 的局部應用與照護可以改善皮膚的光老化。表皮角質形成細胞 (keratinocytes)、黑色素細胞 (melanocytes) 及真皮層纖維細胞 (dermal fibroblasts) 都是視黃酸藥理活性作用的重要標靶細胞。視黃酸可影響黑色素細胞的黑色素生成，其作用是多位點的，對酪氨酸羥化酶(tyrosine hydroxylase)、多巴氧化酶 (dopa oxidase) 及二羥基吲哚氧化酶(5,6-dihydroxyindole-2-carboxylic acid oxidase) 等三型催化酶活性都有抑制作用，從而降低黑色素形成並減輕皮膚的色素沉著。視黃酸亦和細胞分化生長有關，將細胞醣基化並促進組織生長與分化，對於減少細紋和皺紋、皮膚粗糙、改善皮膚紋理已被證明是有效的。視黃酸類藥物已被證實可增加光老化皮膚細胞更新、防止膠原分解與增厚表皮來改善皮膚膚質。然而視黃酸也存在一些缺點，包括對光線和氧氣的不穩定性、在高濃度下具有細胞毒性和對皮膚的刺激。未來的劑型設計與開發以改善上述的缺失對視黃酸於光老化的照護是非常重要的。

The pursuit of a youthful skin appearance is a human nature. It is a long time to pay the attention for preventing cutaneous aging. The environmental exposure of skin has led to the production of acute injury and inflammatory response. The detection of skin aging is based on the change of cutaneous structure and visual appearance. Both intrinsic and extrinsic factors can contribute to the aging of skin. The predominant inducer of extrinsic skin aging is the exposure under solar ultraviolet (UV) irradiation, especially UVA. There are many investigations reporting the efficacy of topically applied retinoic acid (RA) on the prevention and treatment of skin aging. RA can target to keratinocytes, melanocytes, and dermal fibroblasts to exhibit the pharmacological activity. RA functions in melanocytes to suppress the expression of tyrosine hydroxylase, dopa oxidase, and 5,6-dihydroxyindole-2-carboxylic acid oxidase. This signaling inhibits the production of melanin, resulting in the reduction of skin pigmentation. Moreover, RA can regulate the cell differentiation and growth. The glycosylation by RA promotes the skin regeneration and differentiation to minimize the induction of skin furrows, wrinkling, and roughness by photoaging. RA is proved as a drug to increase the skin renewing, prevent collagen degradation, and promote epidermal thickening. However, it should be cautious that RA shows some drawbacks when using for photoaging treatment. These include the instability under light and oxygen, cytotoxicity, and skin irritation. It is important to design the novel formulations for improving these shortcomings in the near future.