Focal myositis (FM) is a rare and restricted skeletal muscle inflammation, presenting as a solid mass with a typical lower leg localization and benign prognosis. In most cases the process solves spontaneously or after immunosuppressant therapy, but sometimes it recurs or progresses to a systemic inflammation. The basis of the disease are mostly unknown. Hence, we provide an update of histopathological features of FM, in order to better define the underlying pathomechanisms of this disorder. A PubMed literature search was focused on the case reports published in English from July 1977 to December 2023. FM and other myositis may show similar morphological features. Emerging studies on MMP molecules and future eventual research on microRNAs (miRNAs) could help in differential diagnosis. Clinical, laboratory, neurophysiological and imaging findings can allow a correct diagnosis. However, muscle biopsy seems to be the only diagnostic tool to differentiate among FM and other localized soft tissue masses.