T-cell lymphomas are a diverse group of rare non-Hodgkin lymphomas. The variety of T-cell lymphomas can be grouped into 2 broad clinical categories: the usually aggressive systemic T-cell lymphomas can be included under the term peripheral T-cell lymphoma (PTCL), and the usually indolent T-cell lymphomas presenting in the skin can be included under the term cutaneous T-cell lymphoma (CTCL). Characteristics of each category can overlap. Systemic T-cell lymphomas or PTCL often present with cutaneous lesions, and CTCL can also present with systemic disease. The most precise way to communicate about these disorders is to use the specific name of the subtype. The most frequently diagnosed form of CTCL, mycosis fungoides (MF), is a primarily indolent malignancy that is most frequently managed using a variety of milder treatment approaches, such as skin-directed therapies or biologic systemic therapies. Among the MF patients who often require a more aggressive approach is a subset who develop transformed MF. Transformed MF is a rare, histologically distinct entity that can be associated with a worse prognosis. Studies are lacking to guide the optimal treatment for these patients; treatment options are generally extrapolated from the algorithms for other aggressive T-cell lymphomas or from the rare patients included in trials of therapies for CTCL. Novel agents already in development or used for MF may offer patients with transformed disease new treatment alternatives.