Heterogeneity within malignant neoplasms, although described for many years by pathologists, has only recently been extensively studied in the laboratory. It is now accepted that most tumors are composed of subpopulations of cells that differ in many phenotypic characteristics including the ability to form a metastasis. Cells with the capacity to metastasize are the ones most likely to prove lethal to the patient since clinicians can often control the primary neoplasm with surgery or radiotherapy. In this report the process of metastasis is discussed, those aspects of tumor cell heterogeneity that are relevant to this process are reviewed, intrapatient tumor heterogeneity is explored, and future preclinical studies are evaluated that may be useful in designing treatment strategies for patients with disseminated malignancies.