Reverse genetic techniques, including gene 'knockouts' and transgenesis, allow defined mutations to be introduced into the mouse genome. The application of these techniques to neurobiology is beginning to provide a bridge between genes and cognition. Specifically, genetically altered mice make it possible to explore molecular mechanisms underlying implicit and explicit forms of learning, short-term and long-term memory, and emotional behaviors. The analysis of these mutant mice has begun to link specific behavioral deficits to defined changes in synaptic physiology.