The theoretical and experimental foundations of some of the most common frequency-domain photothermal techniques for measuring thermophysical properties of materials are presented. Limitations of these methodologies when used without attention to satisfying the appropriate validity conditions are discussed and their consequences in providing inaccurate and often conflicting quantitative measurements are examined in the form of several case studies in photothermal thermophysics. The importance of adherence to experimental setup configurations and signal generation conditions consistent with photothermal theoretical models used to extract thermophysical properties (diffusivity, effusivity, optical absorption coefficient) is highlighted as an essential requirement for reliable thermophysical measurements.