For more than 70 years, implantable intraocular lenses have been used in cataract surgery as a routine replacement for the opaque crystalline lens, preventing postoperative aphakia. Intraocular lenses can also be implanted into the phakic eyeEye to adjust refractionRefraction in cases where corneal refractive surgery is contraindicated. IOLs as supplementary lenses can also be an option for pseudophakic eyesEye to correct for residual refractive error. Such lenses must be selected carefully for each individual patient. There are different options for calculating the refractive power of intraocular lenses for a given patient, based on biometric measures and intended target refractionRefraction. These options include empirical calculation strategies (using a big data approach), formula based calculations (with formulae based on simplified linear Gaussian optics), and (numerical) raytracing. In this chapter we summarise the available strategies for calculating standard stigmatic lenses, toric lenses for correction of corneal astigmatism, and supplementary lenses. The relevant input parameters will be discussed and the fundamentals for extracting formula constants outlined, using methods based on clinical datasets of preoperative biometric measures and postoperative outcome data to tune the results and reduce the calculation prediction error for future cataract surgeries.