In scientific research or routine applications, quantitative measurements are almost indispensable. When the samples contain several compounds of interest or when the compounds of interest are accompanied by other compounds that possibly can influence their measurement, first a separation should be performed, allowing a selective quantification. Several chromatographic techniques (e.g., liquid, gas, supercritical fluid) can be applied for this purpose. Peak areas or peak heights derived from the chromatograms are used as responses to quantitatively compare the compound(s) of interest with reference standards. A necessary step in the quantification procedure is to determine the 100relation between the measured response and the analyte’s concentration, which is called calibration. It is done by measuring the responses of solutions with known concentrations of the compounds of interest, which finally results in a calibration line or curve. If possible, a linear calibration, that is, a straight-line model, is preferred. The first part of this chapter will discuss the classic least-squares straight-line calibration, considering also common mistakes and misunderstandings. The second part will handle some variations on the classical calibrations, such as one-point calibration, calibration models with heteroscedastic data, calibration using internal standards, calibration with standard addition, non-straight-line calibration, calibration after linearization transformation, inverse calibration, orthogonal calibration, and multivariate calibration.