<p>Flash methods such as laser or light flash have developed to be one of the most commonly used techniques for measuring the thermal diffusivity and thermal conductivity of various kinds of solids such as metals, ceramics, polymers, graphite and composite materials. During a flash test, the front side of a plane-parallel specimen is heated by a short laser or light pulse. The resulting temperature rise on the rear surface of the specimen is measured versus time. By evaluating this temperature rise, the thermal diffusivity of material can be determined without requirement for any sensor calibration. Fast measurement times, easy adaption to carrying out temperature-dependent tests, easy specimen preparation, small specimen dimensions, and high reliability are only some of the advantages of this non-contact measurement technique. The method allows also for measurement of the specific heat capacity by a comparative method and therefore, thermal conductivity determination. This requires calibration of the signal height by employing a specific heat capacity standard material with comparable specimen geometries and surface emissivities. Since the introduction of the method in 1961, significant improvements have been made on the hardware, the mathematical treatment of the detector signals and the general instrument operation <i>via</i> the software. Presented in this work is an overview of the working principle, the mathematical background, the general uncertainty of the method, results of comparative tests and some typical application examples for the flash technique.</p>