Sporadic inclusion body myositis (sIBM) is an intractable and progressive skeletal muscle disease of unknown cause that has no curative treatment. Its prevalence varies among countries and ethnic groups. The clinical course is slow and chronic worsening. Diagnosis of sIBM is usually made 5 years after onset. Muscle weakness and atrophy in the quadriceps, wrist flexor, and finger flexors are the typical neurological findings of sIBM. Dysphagia and asymmetric weakness are often found as well. Serum creatine kinase is usually below 2,000 IU/L. Muscle biopsy typically reveals endomysial inflammation, invasion of mononuclear cells into non-necrotic fibers, and rimmed vacuoles, suggesting that inflammation and degeneration are coexist in the pathomechanism. The etiology of sIBM is still unknown; however, genetic factors, aging, lifestyle, and environmental factors may be involved. Recent studies have implicated amyloid beta accumulation, defects of proteolysis, and immune system abnormalities in the pathomechanism of sIBM. sIBM is generally refractory to current therapy, such as steroids or immunosuppressants. Recently, alemtuzumab, which targets T cells, has resulted in improvement in quantitative muscle strength testing. New strategies to induce proteolysis and autophagy, accelerate muscle regeneration, inhibit myostatin, and modulate inflammatory cells are promising. Elucidation of the pathomechanism of sIBM is the key to developing effective therapies.