Transitions between metastable states separated by energy barriers are key to many processes of practical interest in chemistry, engineering, materials science and physics. First-order phase transitions between solid, liquid and gaseous states are amongst the most obvious examples and are critical in such wide-ranging phenomena as the production of pharmaceuticals [1,2], the function of the malaria parasite [3], and cloud and polar cap formation [4,5]. The wetting of both smooth and textured surfaces, the diffusion of adatoms on surfaces and the role of bubble cavitation in damage to marine propellers [6] are just a few of the engineering applications.

The common thread running through these diverse problems is their multiscale nature which creates unique challenges for experiment, simulation and theory. Experimentally, the early stages of these processes - nucleation, wetting, etc. - typically occur at the nanoscale and isolating them is difficult. Recent advances such as the use of cryo-TEM in ref. [2] are beginning to give insight at this fundamental level. In contrast, atomistic and ab initio simulations give direct insight to the nanoscale but the problem for them is that the system sizes and timescales are small, compared to experimental (macroscopic) conditions. This is the case for instance with chemical reactions (typically involving a transition from a higher to a lower energy state separated by a barrier) and associated calculation of the pathways and rates.

To overcome these limitations, a collection of rare-event techniques have been developed over the last 25 years such as the nudged elastic band and string methods among others to study such transitions. At the same time, attention in individual applications has focused on issues such as the definition of unbiased collective variables. Theoretically, the challenge has been that mesoscale approaches (such as the Landau theory of phase transitions) lack molecular-level detail while microscopic tools like classical density functional theory have not been sufficiently robust. However, recent advances in the latter now allow for the molecular-level description of highly inhomogeneous systems, such as wetting of heterogeneous substrates [7] so that, in combination with other frameworks such as fluctuating hydrodynamics, a complete microscopic theory, e.g. of crystal nucleation, is feasible [8].