Abstract: An endoscopic instrument includes an elongate shaft with an optical system arranged therein. The shaft is closed off at least its distal end by a window which is firmly and tightly connected to the shaft. The closure window is constructed of spinel or yttrium-aluminum-garnet. The spinel may be cubic monocrystalline spinel or polycrystalline spinel, and the yttrium-aluminum-garnet may be undoped or neodymium-doped. Similar closure windows of the same materials may be provided at other openings in the endoscope shaft, such as a proximal end ocular or eyepiece or a lateral light guide window. The closure window may optionally be in the form of a lens to function as part of the optical system.