Idiopathic inflammatory myopathies (IIM) are a group of rare, auto immune mediated myopathies, commonly referred to as ‘myositis’. Most subtypes are treatable, except for inclusion body myositis (IBM). Clinically, IIM are characterised by subacute proximal muscle weakness which usually develops within weeks to months. Depending on the subtype, multiple extra muscular organs may be involved. The overarching aim of this manuscript was to evaluate the diagnostic value of currently used tests at diagnosis and their value during the follow-up of patients. In chapter 2 we focussed on the diagnostic value of muscle imaging by evaluating whole body MRI and muscle ultrasound. In chapter 3 we evaluated a multimodality screening strategy for (peri)myocarditis. In chapter 4 the study protocol of the ADAPT study is described. The study is designed to show the diagnostic value of several individual diagnostic tests, and the incremental value of combinations of different diagnostic tests in diagnostic strategies. In chapter 5 we investigated the B cell receptor (BcR) repertoire in muscle biopsies of IIM patients at baseline before treatment, and in peripheral blood samples both at baseline and at follow-up after IVIg treatment. Additionally, we studied the correlation of the BcR repertoire with clinical response to treatment. In chapter 6 we explored the clinimetric properties of a functional disability scale, the Academic Medical Centre Disabiltiy Score (ALDS) in patients with IIM. In chapter 7 the findings of this thesis are discussed and future directives are put forward.