The idiopathic inflammatory myopathies (IIM), classically dermatomyositis (DM), polymyositis (PM), and inclusion body myositis (IBM), are acquired systemic autoimmune disorders defined by chronic muscle weakness and inflammation of unknown aetiology. The combination of clinical, laboratory, electromyographic, and histological features is the basis of diagnosis, as well as exclusion of several mimicking conditions (Bohan & Peter, 1975; Dalakas & Hohlfeld, 2003; Mann et al, 2010; Mastaglia & Phillips, 2002). IIM are the most common causes of acquired muscle disease in adults, but are still rare conditions with an estimated overall prevalence of 50 to 100 cases per million (Oddis et al., 1990; Prieto & Grau 2010; Wilson et al., 2008). In recent years, taking into account additional clinical, immunological and histological features, new phenotypes among IIM, such as antisynthetase syndrome, autoimmune necrotizing myopathy, connective tissue disorder-associated myositis, or cancer-associated myositis (CAM), have been described (Cox et al., 2010; Dalakas, 2010; Dimachkie, 2011; Rider & Miller, 2011; Targoff, 2008). The association between cancer and IIM has been widely reported in the medical literature, particularly in DM patients (Buchbinder et al., 2001; Sigurgeirsson et al., 1992). Cancer screening is a common practice in patients recently diagnosed with IIM, but there is not consensus about how, and how often screening should be performed. The aim of this chapter is to describe the epidemiological, clinical, laboratory, and histological reported features about CAM, to analyze the current potentially approach to preclude malignancy in IIM, and to provide an advisable algorithm in the diagnosis of occult cancer in myositis.