[en] The semi-classical and quantum-mechanical theories of pre-equilibrium processes in nuclear reactions are reviewed, with special attention to the exciton, hybrid and unified models, and the Feshbach-Kerman-Koonin quantum mechanical theory. The results of the 'International Nuclear Model and Code Comparison on Pre-Equilibrium Effects' carried out under the auspices of the NEA Data Bank are presented. The quantum-mechanical theory is described and selected data for pre-equilibrium processes in ⁹³Nb and ⁵⁹Co are analysed using both the semi-classical models and the quantum mechanical theory. The future development of analyses of pre-equilibrium processes is discussed with particular reference to the scopes of the various models and their optimum areas of applicability. (author)