This article presents the historical development of the different generations of zirconia and their range of indications, from veneered to monolithic zirconia restorations. Because of the large extent of this topic, it is divided into two parts. In Part I, the mechanical and optical properties of the three generations of zirconia materials are discussed critically and theoretically. A short summary is given of the current scientific literature, investigating the third generation of zirconia comparatively regarding the properties discussed. Keywords: contrast ratio, flexural strength, full ceramic, monolithic zirconia restorations, reflectance, translucency, transmission, veneered zirconia restorations, zirconia