Mycosis fungoides is the most common form of cutaneous T cell lymphoma caused by monoclonal proliferation of lymphoid tissue cells in the skin. Here we review current literature on the main issues of etiology, pathogenesis, classification, clinical and pathomorphological criteria of the diagnosis of mycosis fungoides, and modern treatments. The disease is difficult to diagnose, especially at early stages. The disease onset presents with a wide variety of clinical symptoms, from complete absence of changes in the local status of a patient to pronounced exfoliation. Histological patterns of biopsy specimens are highly variable and may be non-specific over a long period of time. The classic variant of mycosis fungoides mimics the clinical and histological picture of various groups of chronic dermatoses, especially at early stages. Therefore, it is important to know the main clinical variants of the course of mycosis fungoides in order to identify the groups of patients, who are likely to develop skin lymphomas, provide effective early diagnosis and timely rational choice of therapy for this nosology, and improve the quality of life of patients with lymphoproliferative diseases. The article reports a case of mycosis fungoides, including medical history and description of clinical and histological presentation. Thus, the diagnosis of mycosis fungoides is based on comprehensive evaluation of anamnestic, clinical, and morphological data and the use of immunological and molecular diagnostic methods.