The diagnosis and classification of T-cell lymphomas and NK/T-cell lymphomas based on bone marrow (BM) biopsy findings alone is challenging and controversial. Primary presentation in the marrow occurs in those lymphomas where it is a main and usual site of involvement and rare examples of nodal-type T-cell lymphomas and extranodal T-cell and NK/T-cell lymphomas with isolated marrow involvement or where presentation in the marrow precedes that of extramedullary sites. In the former, there is a lack of consensus on how to classify cases that deviate from established criteria and in the latter, there is often a lack of accepted defining criteria that are applicable to BM biopsies. Therefore, the focus of the 3 session of the bone marrow (BM) workshop of the 19th meeting of the European Association for Haematopathology/Society of Hematopathology in Edinburgh in 2018 was on diagnostic challenges posed by difficulties in clarifying borders between overlapping primary bone marrow T-cell and T/NK lymphoproliferations and the limitations of applying existing defining criteria related to nodal-type T-cell lymphomas and extranodal T/NK cell lymphoproliferations to isolated involvement or initial presentation in the BM.