An algorithm, which combines the scanning beam with dynamic collimation to generate any arbitrary intensity profile, is presented. The desired intensity profile is assumed to be piecewise linear. The dynamic collimation method used is the "sliding window." The algorithm can be used either for a given scanning beam profile or to simultaneously determine the scanning beam profile and the leaf motions required to generate the desired intensity profile, which minimize the total treatment time. The limitations imposed by the physics of an elementary beam are taken into account. The algorithm is an iterative one, with typical calculation times being of the order of a few milliseconds.