Polymyositis, dermatomyositis and related disorders, also known as the idiopathic inflammatory myopathies (IIM), are an uncommon, heterogeneous group of diseases. This is an omnibus protocol designed to continue our description of this rare group of diseases, further delineate important groups of patients, and obtain useful material for further study of immunological abnormalities, pathogenesis and etiology of IIM. Often the diagnosis of an IIM can be confused with other illness (such as adult-onset dystrophies), and therefore, we will be evaluating patients with other illnesses (who are referred with a preliminary diagnosis of an IIM). Patients evaluated with IIM will receive (with informed consent) the best standard therapies available and consideration for entry into any therapeutic protocols. MRI will be performed on healthy volunteers to help develop a mathematical method for quantitating edema of muscle MRI in patients. MRI/PET will be performed on some patients and on healthy volunteers to evaluate its role in understanding muscle disease.