The work presented in this thesis aims to develop a modelling approach to predict crystalline phase evolution in bulk metallic glass during additive manufacturing with laser-based powder bed fusion (PBF-LB). Metallic glasses are non-crystalline metallic materials that generally possess exceptional properties because of its amorphous struc-ture. Manufacturing of metallic glass is possible by rapid cooling of a liquid metal alloy. The required cooling rates to avoid crystallisation generally limits traditional manufac-turing techniques to small/thin samples. The desirable properties of metallic glasses motivate manufacturing of larger samples. PBF-LB is one promising method by which bulk metallic glass potentially can be produced without size limitation. Cooling rates in this process are generally several orders of magnitude higher than critical cooling rates to bypass crystallisation in glass forming alloys. Crystalline structures may still evolve within the solid material because of thermal cycling during the manufacturing process. Numerical simulation can assist development of process for bulk metallic glass formation by predicting the phase evolution. Simulations can also help to increase the understand-ing of where and when crystalline structures develop with respect to process parameters and scanning strategy. Simulation of bulk metallic glass formation during PBF-LB is a challenge. The thermodynamic and kinetic properties of the material and the large variations in time and length scales in the process makes accurate simulations difficult. This thesis address these challenges by developing a modelling approach for simulation of the temperature history and crystalline phase evolution. The objective is to assist the development of process parameters for bulk metallic glass formation. The approach includes finite element modelling to compute the temperature history in the heat affected zone. The modelling includes approximations of the energy input and approaches to sim-ulate the large variations in time and length scales associated with PBF-LB. Computed temperature histories acts as input in calculations of the crystalline phase evolution in the metallic glass. The phase transformation modelling approach includes a modified isothermal model and classical nucleation and growth theory. The result is a coupled thermal and phase transformation model that can predict the trend in crystalline phase evolution in a bulk metallic glass with respect to the process parameters. The predictions show very good agreement to experimental estimates of the crystalline volume fraction. Comparison of simulations makes it possible to evaluate the process parameters in terms of crystalline size distribution. The model is a powerful tool that help the development and fine tuning of process parameters to produce bulk metallic glass.