AbstractUsing the thermal lens experimental configuration, many types of fast molecular dynamics besides the density variation (the normal thermal lens signal) are reflected by the lens signal. Various transient lens techniques in a fast timescale are reviewed. By using the population lens signal, excited state dynamics can be studied with a probe light which has less restricted wavelength range over a wide timescale from a few femotoseconds to seconds. From the optical Kerr lens signal, molecular reorientational dynamics in solution can be revealed. The temporal development of the temperature lens signal provides unique information such as the thermalization time after nonradiative transition from the excited state, which is usually very difficult to determine by other spectroscopies. Identification of the lens components and the advantages of the time‐resolved transient lens method are discussed.