In cataract surgery, the replacement of the opaque lens with an artificial lens is the ideal correction. For the present, the intraocular lens power determination is an essential part of the preoperative evaluation of an eye. The paper presents a short history about the development of implant power calculation, the main formulas used in present, the precise choice of postoperative refraction, the factors affecting accuracy of implant power calculation and some preoperative considerations (patients considerations, intraocular lens characteristics considerations). The final part contains some conclusions about the methods of implant power calculation.