The presence of moisture in a transformer needs to be monitored throughout its service life. Moisture deteriorates transformer insulation by decreasing both electrical and mechanical strength. High moisture content accelerates solid insulation aging, reduces the breakdown strength and makes the transformer vulnerable to the overload conditions due to high temperature spot bubbling. In addition to that, partial discharge can occur in a high voltage region because of the moisture disturbance. Traditional indirect estimation of the moisture concentration in the solid insulation of power transformers includes testing the oil samples as well as measurement of the insulation resistance and loss tangent (50/60 Hz) of the transformer. However, these methods usually give limited information and may lead to wrong conclusions. Direct measurement is not viable and may not be representative to take a paper sample from the surface insulation because moisture distribution is not homogeneous along the insulation geometry. Dielectric Frequency Response, DFR was introduced more than 20 years ago and has been thoroughly evaluated and proven. Several documents have been published summarizing the research work and field tests all over the world. DFR is a practical non-intrusive and non-destructive technique for moisture condition assessment in power transformers, a breakthrough compared with traditional methods. Scientists, researchers and utility operators have shown great interest in the development and application of DFR technique. In this paper, the limitation of traditional methods is presented at first. Later, a comprehensive review of DFR method demonstrates its advantages over traditional methods. Finally, latest research about the mathematical model, temperature correction and test voltage is included to answer common questions regarding the application of DFR method.