Device scale modelling is considered to be one of the most important approaches to the designing of energy efficient thermal energy storage (TES) devices. With increasing computer power, computational fluid dynamics (CFD) simulation of unsteady-state transport phenomena in TES in a well resolved three-dimensional mesh is becoming standard. Detailed understanding of these phenomena through simulations allows engineers/researchers to improve storage efficiency of TES, and optimise the material and operational costs. Scientifically, the device scale modelling provides a vital linkage between the TES material-scale properties and TES system-level performance discussed in various chapters of this book. In this chapter, device scale modelling for various TES technologies, in particular, sensible heat (SHS), latent heat (LHS) and thermochemical heat storage (THS) technologies are considered. SHS and LHS are divided into two main groups, short-term and long-term TES, depending on their storage durations. In the case of short-term TES, the impact of the ambient on the storage performance is considered to be negligible. Therefore, the simulation of the processes within the storage devices with their appropriate boundary conditions is discussed. When the simulation of long-term or seasonal storage devices is discussed, the interaction of storage devices with ambient is considered. Lastly, the simulation of THS for high-temperature TES is discussed.