A stye is an inflamed oil gland or hair follicle on the eyelid. It may occur along the lash line (external) or under the eyelid (internal). Both are swollen, red, and painful, but external styes usually do not last long. Styes are caused by bacteria that normally exist harmlessly on the skin of the eye. Styes often do not require treatment other than warm compresses, but for recurring styes, doctors may recommend an antibiotic. Surgery may sometimes be used to drain a stye. Follow the links below to find WebMD's comprehensive coverage about how a stye is caused, what it looks like, how to treat it, and much more.