The inflammatory myopathies - myositis - encompass a heterogeneous group of chronic muscle disorders of unknown origin and with varying prognoses. New clinical phenotypes of myositis have been identified since the most widely used classification criteria were proposed in 1975. Based on clinical and histopathological features, inclusion body myositis was identified. Furthermore, the myositis-specific autoantibodies may also identify different clinical phenotypes and serve as prognostic markers. The different classifications and inclusion criteria that have been used in different studies make some epidemiological data uncertain. In order to improve our knowledge of causative factors, as well as of pathogenic mechanisms, there is a need for revision and also for an international acceptance of the classification criteria. During recent years, our knowledge has increased regarding the role of some genetic and environmental factors that could affect susceptibility for developing myositis as well as the prognosis. Whether there is an association between myositis and malignancies has been a subject of controversy for many years and recent epidemiological data have brought some clarification on this issue.