Classical Nucleation Theory (CNT) provides a quasi-mechanistic description of the nucleation process, connecting crystallization equilibrium thermodynamics and nucleation kinetics. First applied by Gibbs 150 years ago to describe the formation of liquid droplets from saturated vapours, CNT was developed over the following century and is now often applied to explain the emergence of ordered materials from liquids. In CNT, there are two contributions to the free energy of a metastable system: a volume and a surface free energy both associated with the new, emerging phase embedded in an out-of-equilibrium parent phase. Forming a stable phase is thermodynamically favourable, but the resulting interface, which delimits the new phase from the parent one, carries an energy penalty. While the free energy gain associated with the formation of a stable phase scales linearly with the volume of a nucleus, the free energy cost of forming an interface scales with its surface area. This simple but general argument provides a rationale for the barrier to nucleation and a definition for the transition state associated with the nucleation process .