Glaucoma drainage devices (GDDs) have a fixed place in the treatment of complicated refractory glaucoma. By implantation of an episcleral GDD, a permanent open sclerostomy is formed, through which a silicone tube is placed into the anterior chamber (or pars plana). Thus, aqueous humor is drained out of the eye under the Tenon and conjunctiva, leading the fluid to a base plate made from silicone or polypropylene. Placed near the equator of the eye, this base plate leads to the formation of a cyst that renders resistance to fluid transport, ultimately leading to a final drop in intraocular pressure. GDDs have been mainly used in complicated glaucoma cases that had previous unsatisfactory results from conventional glaucoma surgery. Recent studies suggest that the implants can also be used satisfactorily in early stages of glaucoma. Other devices are positioned and fixed in the chamber angle. Here they either drain aqueous humor into the Schlemm's canal or into the subconjunctival or subchoroidal space. This article gives an overview of the current principles, indications, methods, and possible complications of implantation.