This chapter describes three approaches for 3D hot-spot localization of thermally active defects by lock-in thermography (LIT). In the first section, phase-shift analysis for analyzing stacked die packages is performed. The second example employs defocusing sequences for the localization of resistive electrical shorts in 3D architectures, and the third operates in cross sectional LIT mode to investigate defects in the insulation liner of Through Silicon Vias. All three approaches allow for a precise localization of thermally active defects in all three spatial dimensions to guide subsequent high-resolution physical analyses.