To understand how synapses form, it is important to be able to watch them as they form. Transgenic mice in which motor axons are indelibly labeled with the Green Fluorescent Protein (GFP) or one of its spectral variants (XFPs) provide a new way to image motor nerve terminals; when combined with contrasting stains for the postsynaptic membrane, both pre- and postsynaptic elements can be viewed in live animals. The development, maturation, stability, remodeling and regeneration of neuromuscular junctions and motor units can then be assessed over intervals ranging from seconds to months.