Crystallization is an extremely important unit operation the food,chemical,and pharmaceutical industries,among others.However,understanding the molecular-level details of nucleation and polymorph selection in molecular crystals remains a challenging problem.On the one hand,the stochastic nature of nucleation,and the fact that is usually involves only a few molecules,make it very hard to probe it experimentally.On the other hand,the time scales involved are well beyond the reach of traditional molecular simulation methods.In this work I present recently developed methods to model the nucleation of molecular crystals,both from the melt and from solution.The methods involve a combination of the string method in collective variables [1],milestoning using Voronoi tesselations [2],and a generalized set of order parameters described in [3].I will also address some of the main challenges in using these methods to describe nucleation in multicomponent systems.