Due to their rarity, heterogeneity, and multispecialty nature, the myositis syndromes have limited data-driven consensus on appropriate outcome measures. Recently, two international, multispecialty consortia developed new tools and consensus on core set measures of myositis disease activity and damage, as well as response criteria that are now recommended for use as clinical trial endpoints but will also be useful in clinical practice. Magnetic resonance imaging, muscle ultrasound, selected laboratory tests, and immunological biomarkers—including cytokines, chemokines, lymphocyte flow cytometry, and endothelial activation markers—can all be helpful adjuncts to serum muscle enzyme levels in assessing disease activity and damage, but these have not yet been fully validated. Definitions of clinically inactive disease, complete clinical response, and remission have also been proposed but require further validation. These advances should enhance the development of therapies by standardizing our ability to demonstrate their efficacy in treating the idiopathic inflammatory myopathies.