Phase-change materials have a wide range of optical and electrical applications due to a unique combination of structural, electronic and optical properties. The phase transition between the crystalline and amorphous phases is a central physical process in devices based on phase-change materials. In order to design and to develop future applications with novel functionalities we need to understand on the nanometre length scale and the (sub)nanosecond timescale the electrical, thermal and phase-transition behaviour of phase-change materials. At the nanoscale the phase-transformation in phase-change materials is governed by the chemical composition, atomic structure, and the input energy due to temperature (Joule heat), electric field and/or electronic excitations. Therefore, a successful phase-change model should capture all these elements and predict the phase information as an output. We discuss the theoretical models that have been used to study and predict the crystallization of phase-change materials that span the materials modelling spectrum from electronic/atomistic simulations (atomic behaviour) to microscopic and continuum models (bulk behaviour). Real applications and device design require the use of microscopic and continuum models due to large computational times of atomistic simulations. On the other hand the predictions provided by microscopic and continuum models depend on the chosen parameter set used to describe the phase transition. We stress the importance of ‘building bridges’ between atomistic and continuum modelling to design future phase-change devices with novel and enhanced functionalities.