This chapter focuses on cognitive as opposed to sensori-motor skills and on models that create or alter symbolic knowledge representations. It deals briefly with models that learn by adjusting quantitative properties of knowledge structures. Although occasionally referring to empirical studies, the chapter is primarily a review of theoretical concepts. It proceeds on the assumption that each hypothesis contains some grain of truth to be extracted and incorporated into future models. The learning mechanism is a more finegrained unit than the model or the cognitive architecture. Cognitive descriptions of processes in the mind are functional descriptions of what this or that piece of wetware is doing, what function it carries out. This perspective points to the need to understand the relation between learning mechanisms and modes of neural plasticity.