Forward flux sampling (FFS) is a computational method used to study rare events in complex systems. Unlike TPS and TIS, FFS does not require the system to be at equilibrium, and can thus be applied to out-of-equilibrium processes. Similarly to TIS, FFS is based on sampling transitions between non-intersecting interfaces defined in a low-dimensional order parameter space describing the transition from a reactant state, 𝐴, to a product state, 𝐵.