Our appreciation of the relationship between synaptic structure and function, and in particular our understanding of quantal synaptic transmission, is derived from classical studies on the neuromuscular junction. However, physiological studies of quantal transmission at mammalian CNS synapses have produced a variety of results, and thus no consensus of opinion has emerged. This variability could be due, in part, to experimental and analytical limitations or to differences in the structural and functional features of central synapses, or both. Some of the experimental limitations have recently been overcome by the use of novel preparations that permit direct measurement of quantal synaptic events in the CNS. Although these studies reveal similarities between the synaptic mechanisms of the neuromuscular junction and CNS synapses, important differences and specializations are also evident. The purpose of this review is to highlight the structural and functional diversity of synapses in the mammalian CNS, and to discuss the potential relevance of structural features to synaptic function.