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A new treatment technique, intensity modulated arc therapy (IMAT), has been implemented in the authors' clinical practice. Instead of delivering intensity-modulated beams with fixed gentry angles, IMAT delivers optimized dose distributions using rotational beams. During delivery, the field shape, which is formed by a multileaf collimator (MLC), changes constantly. Treatment plans are developed with a combination of forward and inverse planning. Arcs are approximated as multiple shaped fields spaced every 10 degrees around the patient. Multiple coplanar or non-coplanar arcs are used in combination with wedges. The beam weights can be optimized outside of the commercial planning system. A leaf-sequencer is used to convert the fields from the final plan into MLC leaf sequences. Comparisons were made with conventional and inversely optimized treatment plans. Dosimetric accuracy of the entire process was verified with phantoms before IMAT was used clinically. Patient-specific verifications were performed using both the absolute and relative dose measurements in a humanoid phantom. The authors have demonstrated the clinical feasibility and accuracy of IMAT treatments on patients with head and neck cancers. Generally, no more than 3 arcs are needed to achieve a conformal dose distribution. The complexity of the IMAT is not significantly greater than existing are therapy from the point of view of the operator. The treatment time using IMAT is similar to conventional treatments. Clinical examples of IMAT treatments will be presented to illustrate the dosimetric advantage of rotational delivery.