Tretinoin (all‑trans retinoic acid) is a retinoid used topically for acne and photoaging and orally for acute promyelocytic leukemia under specialist care. In skin, it binds nuclear retinoic acid receptors (RAR/RXR), modulating gene expression to normalize keratinization, increase epidermal turnover, and stimulate new collagen while limiting UV‑induced matrix metalloproteinases. Clinical studies show improvements in fine wrinkles, mottled pigmentation, and texture with sustained use over months. Topical formulations include creams, gels, and lotions, with stability considerations when combined with benzoyl peroxide unless microencapsulated. Use in pregnancy is generally avoided due to limited systemic safety margin.