Physical and biological aspects led to the realization of cancer therapy with pi ** minus mesons. The pions are produced by collision of the protons with target material. They are focused into the treatment volume. Two methods are developed to scan this volume in a 3-dimensional way. The dosimetric system has to take into account that the dose contains spatially LET contributions. CT-slices of the patient serve as a basis for therapy planning. As experience grew, deeper lying tumors were treated. The expectations of the project have recently been accomplished.