<p>The understanding of processes in heat storage materials and reactors can be greatly improved by the use of non-destructive methods that allows the view inside the objects. The advantage of non-destructive methods is that the sample of interest remains intact, experimental changes can be monitored in-situ, and the experiments are less labor intensive. Alongside others, three of the most utilized non-destructive techniques for heat storage systems are discussed: NMR, X-ray imaging, and neutron imaging. The working mechanism and (dis)advantages of these techniques are discussed alongside various applications and examples. This work aims to provide a handle to researchers working in the field of thermal energy storage on how to investigate heat storage materials and reactors in a non-destructive manner.</p>