The neuromuscular junction (NMJ) forms the communicative link between motoneurons and muscle fibers. The properties of motoneurons and muscle fibers are matched in different types of motor units, which are recruited selectively to accomplish various motor behaviors. Motor units and muscle fibers can be classified based upon structural and functional properties, reflecting the essential match between motoneuron and muscle fiber. Using a three-color immunofluorescence technique combined with confocal microscopy, we examined the three-dimensional structure of pre- and postsynaptic elements of NMJs on different fiber types in the rat diaphragm muscle. On type I and IIa fibers, comprising slow-twitch and fast-twitch fatigue-resistant motor units, the structure of NMJs is far less complex than on type IIx6 and IIb fibers comprising fast-twitch fatigue-intermediate and fast-twitch fatigable motor units. We also found a greater extent of overlap between pre- and postsynaptic elements of NMJs on type I and IIa fibers. This review focuses on these normal phenotypic differences in NMJ properties and on the adaptations that occur under various conditions of altered use.