Polymyositis and dermatomyositis are chronic inflammatory muscle disorders with frequent involvement of other organs hence outcome measures should include these different aspects of disease. Muscle strength and muscle endurance are the most specific clinical features that should be assessed during treatment and longitudinal follow-up. Extramuscular involvement should also be assessed. An international, interdisciplinary network, the International Myositis Assessment Clinical Study Group (IMACS) has proposed a core set of outcome measures to assess three dimensions of myositis disease; disease activity (MYOACT), disease damage (MYODAM) and health related quality of life (SF-36) to be used in clinical trials. These include scoring of extramuscular involvement (skin, lungs, articular, cardiac, gastro-intestinal tract) in both the disease activity and damage scores. In the disease activity score, muscle strength is measured by the manual muscle test (MMT)- 8, this could easily be used in clinical practice. Other myositis specific outcome measures are the Functional Index of myositis (FI) -- 2 to measure muscle endurance and a questionnaire, the Myo-sitis Activities Profile (MAP) to measure patient perspective. A close collaboration between physicians, physical and occupational therapists and specialized nurses is of great value in care and disease assessment of patients with polymyositis and dermatomyositis.