The system of Multicellular Tumor Spheroids (MTS) as an additional in vitro model, has been broadly used in cancer research since the 70's to study the characteristics of solid tumors and the effects of anti-cancer agents, particularly of radiation. This paper reviews and evaluates the method according to the authors' experience in using MTS in experiments. MTS simulates conditions found in those solid tumors which have similar histological structures, such as the existence of hypoxic and/or necrotic areas and the manifestation of a decrease of growth fraction as they increase in size.,Various assays with samples in detail have been developed and adopted in MTS experiments including many of the techniques used in tumor biology to determine the dose response of tumors to radiation such as volume change, cell survival, cell viability, histology, cell loss, and cell cycle kinetics. Cells in MTS grown in close contact had an enhanced capability to accumulate and repair the damages induced by treatment, representing a higher resistance to treatment than monolayer cells. TO certain extent the MTS system, being biologically somewhere between in vivo and conventional in vitro monolayer cells, is an attrative modeldue to its low expense high turnover speed and copvience for use.