The role of radiation therapy (RT) in cancer management has grown remarkably during the past few decades. For successful RT, the most important factor is highly conformal radiation delivery focused to the tumor-bearing region while minimizing the surrounding normal tissue damage. RT has evolved from 2-dimensional and 3-dimensional techniques to intensity modulated RT (IMRT) along with the development of computer science and mechanical engineering. IMRT is a special form of 3-dimensional conformal RT techniques, by which the intensities of radiation coming though the beamlets are modulated so that conformal radiation dose distribution around the tumor-bearing region is achieved. IMRT has been covered by the Korean health insurance system since July 2001. The basic principles and process of IMRT are introduced.