In this paper we review the main aspects, both instrumental and clinical, of gonioscopy. This important semeiotic technique allows the ophthalmologists to explore and evaluate normal and pathological iridocorneal angle. Direct and indirect gonioscopic lenses and the complexity of iridocorneal angle's structures are described. Different kinds of glaucoma (trabecular, narrow angles and angle closure, malformative glaucomas) are better analyzed and studied by means of gonioscopy, not only for diagnostics ends, but also for an exact choice of surgical or para-surgical methodologies in the therapeutic time.