Congenital myasthenic syndromes (CMS) are characterized by congenital defects in the neuromuscular signal transmission and are caused by pathogenic variants in 36 genes. Recently identified forms of CMS include TOR1AIP1-CMS, CHD8-CMS, PURA-CMS, and TEFM-CMS. Most forms of CMS are caused by autosomal recessive variants, whereas four forms of CMS are caused by autosomal dominant variants, in which adult-onset cases are not rare. As myasthenic features are not always observed and muscle hypotrophy is sometimes observed, CMS should be considered in differential diagnosis of congenital myopathies and other neuromuscular diseases. Low- and high-frequency repetitive nerve stimulation is essential to diagnose CMS for patients who develop muscle weakness at less than 2 years of age. Tubular aggregates are observed in muscle biopsy in four forms of CMS, and serum CK levels are elevated in some forms of CMS. As rational therapies are available for most forms of CMS, identification of causative gene variants by genetic analysis is required.