The congenital myasthenic syndromes (CMS) are a group of rare genetic conditions characterized by abnormal neuromuscular transmission. Typically, these conditions have been the result of a dysfunctional protein that is present in the presynaptic terminal, the synaptic cleft, or the postsynaptic terminal. Many of these syndromes present within the first few years of life with fluctuating and fatiguable weakness in a distribution similar to myasthenia gravis, although a limb-girdle distribution and late onset are also seen in certain specific types of CMS. Electrodiagnostic testing with repetitive nerve stimulation may be helpful in some forms of CMS.