The introduction of negative pions into cancer therapy has required the construction of large new proton accelerators together with special magnetic systems to form and direct the pion beam to a patient. A summary is presented of the fundamental properties of pions and of the methods used to study the therapeutic beams. The dosimetry of these beams requires the use of the older techniques as well as new methods for determining the different LET components. The data for a number of beams is given and the utilization of this data in treatment planning is reviewed. An important problem for therapy is the behavior of inhomogeneities in the pion beam, and experimental methods are described which illuminate this problem. The studies of the effects of inhomogeneities in a beam point the way toward fruitful comparisons with the computerized treatment planning codes known as PION‐1 and PIPLAN. A useful step in treatment is the verification of doses in patients during therapy. For this purpose the new methods for measuring the high LET doses in patients are described as well as a timing measurement for checking the stopping effect of the tissues as obtained from the CT scans.