In this paper, a review of a resonator's quality factor (Q-factor) measurement is given. The presentation is based on a general Q-factor measurement system so that one can have an overview of such a measurement system: from excitation of a resonator to acquisition of signal resulting from the vibration of the resonator till the signal processing to get the Q-factor. From the way in which the excitation and the acquisition are performed, as well as the way in which the signal processing is done, three major approaches can be distinguished: frequency domain, time-frequency domain and time domain. An overview of the advantages and the limitations of each approach are given with a focus on time domain measurement for it offers the only way to conduct an in-situ Q-factor measurement. Two time-domain design examples are also illustrated.