Two-phase flow is to be found in many situations of nuclear reactor operation and accident sequences. Examples are loss of coolant accidents and boiling in fuel assemblies. Hence, modelling of two-phase flow is a primary concern in nuclear safety research. Model development and code validation frequently require experimental data, preferably taken under plant thermal-hydraulic conditions. Tomographic imaging techniques provide a way to analyze two-phase flow with high spatial resolution. In this paper we introduce recent developments in X-ray tomography and its application to three different problems related to nuclear safety.