Sebaceous cysts are noncancerous, slow-growing bumps that can appear on your skin. While often referred to as sebaceous cysts, these cysts are more accurately called epidermoid cysts because they contain a protein called keratin, not sebum (an oily substance produced by sebaceous glands). Sebaceous cysts typically present as painless, smooth bumps under the skin, often on the scalp, face, neck, torso, or back. They may range in size from small to several centimeters. In rare cases, they may become inflamed, red, tender, or filled with pus.