Withanolides are a group of naturally occurring steroidal lactones primarily found in plants of the Solanaceae family, notably Withania somnifera (ashwagandha). They feature an ergostane skeleton with a characteristic δ-lactone ring in the side chain. These compounds exhibit diverse bioactivities including anti-inflammatory, adaptogenic, neuroprotective, and potential anticancer effects through modulation of NF-κB, Nrf2, and apoptosis pathways. They are lipophilic secondary metabolites biosynthesized from cholesterol via the mevalonate pathway. Concentrations vary by plant part, with roots and leaves being richest sources. Used traditionally in Ayurvedic medicine and increasingly in modern nutraceuticals for stress reduction and immune support.