Using the human tumor colony assay, the growth and chemosensitivity of clonogenic tumor cells present in fresh biopsies of human tumors can be investigated. Excellent evidence has been obtained that colonies grown in HTCA are comprised of tumor cells, and that clonogenic cells within tumor colonies have self-renewal properties (the defining feature of tumor stem cells). Clinical correlations have been made between in vitro chemosensitivity and the response of patients with metastatic cancer to chemotherapy. In a series of trials, HTCA has had a 71% true positive rate and a 91% true negative rate for predicting drug sensitivity and resistance respectively of cancer patients to specific chemotherapeutic agents. HTCA has also had several areas of application to new drug development and screening. Ongoing developmental research is needed to improve growth rates for many tumor types and to further improve assay methodology and thereby enhance its applicability to predictive drug sensitivity testing.