We review the mechanisms that shape the spatial distributions of energetic electrons trapped in the magnetospheres of Jupiter, Saturn, Uranus and Neptune. To determine what controls the energy and spatial distributions throughout the different magnetospheres, we compute the time evolution of particle distributions with the help of a diffusion theory particle transport code that solves the governing 3-D Fokker-Planck equation. We discuss the processes already accounted for in our physics-based models of the outer planet electron radiation belts but also those suspected to be missing to improve our model results. Our theoretical modeling is guided by the analysis of particle, field and wave data collected by Pioneer 10&11 and Galileo at Jupiter, Cassini at Saturn, and during Voyager 2’s flyby of Uranus in January 1986 and at Neptune in August 1989.