In accelerator physics strong focusing or alternating-gradient focusing is the principle that, using sets of multiple electromagnets, it is possible to make a particle beam simultaneously converge in both directions perpendicular to the direction of travel. By contrast, weak focusing is the principle that nearby circles, described by charged particles moving in a uniform magnetic field, only intersect once per revolution.

Milton Stanley Livingston was an American accelerator physicist, co-inventor of the cyclotron with Ernest Lawrence, and co-discoverer with Ernest Courant and Hartland Snyder of the strong focusing principle, which allowed development of modern large-scale particle accelerators. He built cyclotrons at the University of California, Cornell University and the Massachusetts Institute of Technology. During World War II, he served in the operations research group at the Office of Naval Research.