Epimedium, also known as barrenwort, bishop's hat, fairy wings, horny goat weed, or yin yang huo (Chinese: 淫羊藿), is a genus of flowering plants in the family Berberidaceae. The majority of the species are endemic to China, with smaller numbers elsewhere in Asia, and a few in the Mediterranean region. Epimedium species are deciduous or evergreen hardy perennials. The majority have four-parted "spider-like" flowers in spring. The plant contains icariin, which is a weak PDE5 inhibitor in vitro. Its clinical effects are unknown.