Synaptic plasticity is the experience-dependent change in connectivity between neurons that is believed to underlie learning and memory. Here, we discuss the cellular and molecular processes that are altered when a neuron responds to external stimuli, and how these alterations lead to an increase or decrease in synaptic connectivity. Modification of synaptic components and changes in gene expression are necessary for many forms of plasticity. We focus on excitatory neurons in the mammalian hippocampus, one of the best-studied model systems of learning-related plasticity.