Microscopic infrared (IR) thermography has been used in steady-state mode for failure analysis since many years. However, its temperature resolution is limited to the order of 100 mK, and the emissivity contrast complicates the interpretation of results. In lock-in thermography (LIT), the power dissipation in the IC is periodically pulsed, the device is imaged by an IR microscope, and the incoming IR images are numerically processed and averaged on-line, according to the lock-in principle. After an averaging time of typically 30 minutes, T-modulation amplitudes below 100 µK are detectable. This reduces the detection limit for local heat sources, compared to traditional IR microscopy, by a factor of 100 and thus greatly expands the application field of LIT in failure analysis of ICs. Lock-in thermography with two phase detection allows to display images, which are inherently free of the emissivity contrast. By using a solid immersion lens, a spatial resolution of 1.5 µm may be obtained.