Time Domain Thermoreflectance (TDTR) allows precise determination of thermal conductivities and interface resistances in thin films and nanoscale materials. Using an ultrafast, laser-based measurement technique, even extremely small heat flows can be resolved in time and material-dependent properties can be clearly characterised. TDTR is therefore a key tool for understanding the thermal behaviour of modern micro- and nanostructures and optimising them for high-performance, reliable electronic systems.