The technique of using the “particle-in-cell” method was developed in the field of plasma physics [1]. Borrowed for problems of intense beams, it becomes an especially powerful tool because such problems frequently use single species “plasmas” and so pose a less severe requirement on the computer. We will look at several problems in which the PIC code method has been useful. The first is the classical Pierce gun in a transient or short pulse mode. Here the transverse beam optics is strongly affected by the time dependence of the current. The second is a study of high power klystrons searching for the source of an instability. The third is the high power rf source called the “lasertron” which is under development at SLAC. The interesting new development for the lasertron simulation is the introduction of a double gap output cavity for improved efficiency. The lasertron and klystron simulations are steady state solutions to rf problems with high-Q cavities. In order to limit the computation to a realistic time, these simulations use an external equivalent circuit which can communicate with the beam tunnel through ports placed at the locations of the rf cavities. Application for electron beams generally require using a fully relativistic electromagnetic code such as MASK. In some applications, the computation can be speeded up by limiting the solution of the fields to the electrostatic conditions. This can be especially helpful if the degree of precision required demands very large numbers of macroparticles. As a last example, we will show the use of this technique for a problem involving emittance growth for a high intensity beam for heavy ion fusion.