External beam radiation therapy has changed dramatically during the past decade. Intensity Modulated Radiotherapy (IMRT) represents one of the most important technical advances. IMRT allows dose escalation with potential improvement in local control and less toxicity to normal tissues. It's also a radical change in practice for the physicist but particularly for the radiation oncologist. In this article, we have been described the physical and technical principles of IMRT. Indications are reviewed and aiming to summarize the published data. At last, importance of rigorous and reproducible quality assurance procedures should be emphasized and reminded. The potential clinical applications of IMRT are broad and continue to expand but enthusiasm for IMRT does not have to mask reality. Undesired or even potentially dangerous results may occur and no clinical data are currently available to support its use. More long term data and prospective controlled trials are required to confirm the clinical benefits of IMRT.