Myasthenia gravis (MG) is an autoimmune disorder characterized by a defective transmission of nerve impulses to muscles leading to muscle weakness and fatigability. Some, but not all, muscles are affected and not necessarily symmetrically. Increased weakness with continued muscle activity represents a diagnostic clue for MG, but these clinical features can vary. MG is B-cell mediated, and is associated with antibodies directed against the acetylcholine receptor (AChR), muscle-specific kinase (MUSK), lipoprotein-related protein 4 (LRP4), or agrin in the postsynaptic membrane at the neuromuscular junction. Patients should be classified into subgroups to help with therapeutic decisions and prognosis. Subgroups based on serum antibodies and clinical features include early-onset, late-onset, thymoma, MUSK, LRP4, antibody-negative, and ocular forms of myasthenia gravis. Agrin-associated MG might emerge as a new entity. The prognosis is good with optimum symptomatic, immunosuppressive, and supportive treatment. The evolution of MG is unpredictable, but it is generally characterized by the occurrence of relapses, sometimes subsequent to remissions and a worsening trend. For 85% of MG patients, the maximum severity is reached within less than 3 years.