Crystal nucleation from solution is a collective process of assembly that involves, by definition, an ensemble of growth units (molecules, ions, atoms, particles ...) that are inherently equivalent and that come together to form a nucleus of a new phase characterised by a well-defined structural arrangement. In order to describe and ultimately understand the salient features of the assembly process, it is necessary to develop low-dimensional descriptors of the system characterising the transformation. In the theoretical and computational literature on phase transitions, such descriptors take the name of order parameters (OPs). In the context of enhanced sampling, OPs fall into the broader category of Collective Variables (CVs, often indicated as 𝐬(⃗ 𝐫)). These are usually functions of the atomic coordinates that are used to define the bias potential added to the Hamiltonian in biased enhanced sampling or that are used to mark the progress between reactants and products in unbiased enhanced sampling and MD. Very simple CVs to monitor the emergence of crystalline order include the average coordination number, system potential energy and local density, though the correct choice of CVs will depend upon the simulated system. In addition, these simple examples may not be able to distinguish between different crystal polymorphs, in which case some characterisation of the local symmetry between coordinated monomers in crystal structures should be captured by the (combination of) CVs.