This chapter describes the diagnosis, treatment, and prognosis for non-Hodgkin's lymphoma (NHL) in pregnancy. Indolent NHL includes follicular lymphoma and chronic lymphocytic leukemia/small lymphocytic lymphoma, which are extremely rare during pregnancy. Aggressive NHL, which includes large B-cell lymphomas, mantle cell lymphoma, and mature T-cell and NK-cell neoplasms, represents the majority of NHL cases diagnosed during gestation. Due to the aggressive course of these lymphomas, most patients should be treated promptly with intensive combination chemotherapy. Aggressive NHL group includes precursor (B or T) lymphoblastic leukemia/lymphoma and Burkitt's lymphoma. Most chemotherapy regimens for very aggressive lymphomas include high-dose methotrexate, which among the currently used anticancer drugs, poses the greatest risk to the developing fetus when administered during the first trimester. Placental involvement in pregnancy associated NHL is extremely rare, but a single case of dissemination to the fetus has been reported.