Experimental evidence indicates that the physics of molecular collisions is characterised by an underlying simplicity when viewed from the perspective of the dynamics of the nuclei in a particle representation of the species involved. We review this evidence and describe a simple, transparent kinematic theory of inelastic collisions from which the quantum state-resolved outcome of atom-molecule and molecule-molecule collisions are predicted. The principal mechanism of change is linear-to-angular momentum conversion via a torque-arm of molecular dimension constrained to operate within boundaries set by the quantisation of molecular eigenstates and by overall energy conservation. The mechanism is unchanged throughout the wide variety of processes molecules undergo but is modified in a process- and system-specific manner by boundary condition changes and this gives the wide variety of outcomes seen experimentally. The mechanism and boundary conditions may be represented in velocity (momentum)Ðangular momentum diagrams that illustrate vividly the interplay of momentum and energetic factors. Rapid, accurate calculation routines based on these principles reproduce rotational and vibrational distributions observed in experiment over the wide range of inelastic processes that diatomic molecules undergo as well as in atom-diatom reactive encounters.