This chapter contains sections titled: Introduction, General And Stochastically-Independent Stage Models, The Alternate-Pathways (Ap) Model, The Mcclelland-Ashby Cascade Model, Four Experiments, Additivity Of Factor Effects On The Mean, The Summation Test: Further Support For The Stage Model, The Mixture Test: Further Evidence Against The Alternate-Pathways Model, Evidence Against The Cascade Model, Discussion, Appendix: Proofs And Comments On Equations, Notes, References