Myasthenia gravis (MG) is an autoimmune disorder caused by autoantibodies to the postsynaptic acetylcholine receptors (AChR) or to muscle specific kinase (MuSK) at the neuromuscular junction. Autoantibodies are responsible for a defect in neuromuscular transmission, causing the characteristic fluctuating weakness and exhaustibility in voluntary muscles. Characteristic clinical features of MG are changes in severity of weakness over brief periods. The muscle weakness is typically worsening after prolonged use of affected muscles and also as the day goes on. Usually, MG begins with few isolated signs and extends to other muscles within a few weeks to a few months. Ocular, facial, bulbar, shoulder, neck, trunk, limb and respiratory muscles may be involved in varying combinations and degrees of severity. Ptosis and/or diplopia are the most common initial symptoms in about 50 % of patients. Difficulty in chewing, swallowing, or talking is the initial symptom in 15-20 % of patients. Weakness and exhaustibility of proximal limb muscles is a presenting symptom in 15 % of patients. In about 10-15 % patients the initial symptoms are due to weakness of single muscle groups, such as neck extensors, hip flexors, finger extensors or ankle dorsiflexors. The course of MG is variable, but commonly progresses over the time. An early diagnosis of MG and early optimal immunotherapy are of crucial importance for prognosis of MG patients. With longer MG duration the extent of irreversible structural changes of postsynaptic end-plate and AChR becomes greater and, at the same time, their natural regenerative abilities are limited. Therefore, in cases of late MG diagnosis and late MG treatment the possibility to achieve remission or marked improvement is significantly lower in comparison to initial MG stages. The diagnosis of MG should be strongly suspected by history of fluctuating muscle weakness and exhaustibility, which are usually increasing as the day goes and on the muscle exertion. Physical examination of patients with suspected MG must be performed by the means, including specific tests, of detecting varying weakness in specific muscle group. Diagnostic procedures that confirm the diagnosis of MG are intravenous administration of neostigmin, EMG and presence of autoantibodies to AChR or to MuSK.