A variational formulation of the breakdown criteria for plasmas subjected to spatially nonuniform electric fields is developed and applied to several examples. The method uses a different variational function than that developed previously by Epstein. The method is valid for both pulse and cw breakdown. Several examples are presented which illustrate the theory. Application of the formulation to breakdown in front of a narrow infinite slot radiator on a ground plane demonstrates that an effective diffusion length can be defined which is a function of the aperture width.