RecruitingNatural History of Inflammatory Muscle DiseasesObjective: To collect information and biospecimens (such as blood, muscle, and skin samples) that will be used to research causes and treatments of inflammatory muscle diseases. Eligibility: People aged 12 years and older with suspected or confirmed inflammatory muscle disease. Healthy volunteers aged 18 years and older are also needed. Design: Participants will have at least 1 clinic visit. Each visit will last 4 to 8 hours. Some may return for additional visits. All participants will undergo these procedures (unless they are unable to): * Physical exam, including blood and urine tests. * Magnetic resonance imaging (MRI) scan of the thigh. Participants will lie still on a table with padding around 1 thigh. The table will slide into a tube. The scan will last for approximately 40 minutes. Some procedures are optional: * Muscle biopsy: An area of skin will be numbed. A quarter-inch cut will be made. Several pieces of muscle tissue, about the size of grains of rice, will be removed. * Skin biopsy: An area of skin will be numbed. A piece of skin about a quarter inch in diameter will be removed. * Lymph node biopsy: A small sample of fluid or tissue from a lymph node will taken to check for infection. * Genetic testing. Some of the samples collected may be used for genetic testing.