A modular interspinous fixation system and method are shown. The system and method comprise a first plate, a second plate and a locking cross bar for locking the first and second plates together and into locked engagement with a first spinous process and a second spinous process. A bridge is mounted or received on the locking cross bar so that it becomes locked onto the locking cross bar when the locking cross bar is in locked engagement. The bridge and locking cross bar are adapted so that the bridge does not rotate about its axis while on the locking cross bar. Various other embodiments are shown illustrating various bridges of different shapes and sizes, integral and non-integral locking cross bars and even a ramped locking cross bar for facilitating preventing separation of the first and second plates after they are mounted to the first and second spinous processes.