The nonhematopoietic tumors most often diagnosed in the bone marrow are metastatic. Currently, accurate diagnosis of tumor metastasis requires integration of the clinical findings; morphological features; and results of immunohistochemical stains, cytogenetics, and molecular studies. This review focuses on a practical approach to the diagnosis of metastatic tumors in the bone marrow according to current standards of practice and discusses recent advances in understanding of tumor metastasis and the interaction between tumors and the bone marrow environment.