The magnetron has attracted much attention lately as a means of efficiently generating high-power electromagnetic radiation. However there is at present no complete theoretical description of this device. As a step in the development of such a theoretical description simulations of A6 magnetron operation have been performed with the 2-D fully electromagnetic codes MAGIC and MASK. Simulations with MAGIC have revealed mode competition between the ir and the 2irmodes with other operating parameters an mode (5. 0 GHz) is observed. Also density profiles are seen to depart markedly from Brillouin flow during preoscillation and large RF fields are observed at saturation accompanied by energetic bombardment of the anode. Simulations with MASK exhibit the ir-mode and the 2ir-mode separately. The dominant mode is seen to depend on the cathode radius in agreement with experiment. 1 .