Natural killer (NK) cell lymphomas and leukemias are aggressive neoplasms. Clinically, they can be classified into nasal, non-nasal and lymphoma/leukemia subtypes. Treatment results are unfavorable. High-dose chemotherapy and hematopoietic stem cell transplantation (HSCT) may improve patient outcome. For autologous HSCT, a critical review of the literature shows that most patients with nasal NK cell lymphoma in complete remission (CR) appear to do well without HSCT. However, patients with refractory diseases and untreated relapses have poor outcome with HSCT. Therefore, identification of patients with nasal NK cell lymphoma in CR who are at high risk of relapse may be necessary before autologous HSCT can be recommended. Patients with disseminated nasal NK cell lymphoma, non-nasal NK cell lymphoma and NK cell leukemia have poor outcome with autologous HSCT. Allogeneic HSCT may be beneficial to these patients. Most of the reported cases have been performed from HLA-identical sibling donors, and data on alternative HSC sources including matched unrelated donors and umbilical cord blood are very limited. Continuous efforts should be devoted to risk stratification for identifying high-risk individuals for HSCT, and defining the optimal conditioning regimen for NK cell lymphomas.