Microscopic infrared (IR) thermography, as well as liquid crystal imaging and fluorescent microthermal imaging, have been used in steady-state mode for failure analysis since many years. However, the temperature resolution of these techniques is limited to the order of 100 mK. A special problem of IR microscopy is the very inhomogeneous IR emissivity of the IC surface, which needs to be compensated properly. In lock-in thermography (LIT), which is also called Stabilized Thermal Imaging, the power dissipation in the IC is periodically pulsed, the surface of 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 a certain averaging time of some minutes to 1/2 hr, T-modulation amplitudes below 100 µK are detectable. This reduces the detection limit for local heat sources, compared to traditional IR microscopy and other thermal methods, by a factor of 100 and thus greatly expands the application field of thermal failure analysis of ICs. Moreover, the lateral heat diffusion, which leads to a considerable blurring of all thermal images in micro-regions, is considerably reduced by the dynamic character of LIT. Finally, 2-phase lock-in thermography allows to display images being by nature free of the emissivity contrast, which usually disturbs IR microscopy images of ICs. By using a solid immersion lens, the spatial resolution of LIT may be improved close to 1 µm. Lock-in thermography for IC failure analysis is available from several vendors.