Despite its well-known limitations, diffusion remains an important con cept for modeling radiation belt energization and transport. Recent work at AFRL has applied radial diffusion to observations of: (a) 1 -10 MeV equatorially mirroring protons at 1.2 < L < 3, before and after the March 1991 magnetic storm; (b) ~ 1 MeV electrons at 3.5 < L < 6 following the 9 October 1990 storm; (c) 1.6 -3.2 MeV electrons at L = 4.2 between Decem ber 1994 and September 1996, following enhancements of > 2 MeV electrons at geosynchronous orbit associated with high speed solar wind streams. In general, encouraging results were obtained, but only after implementing: (1) non-steady state initial conditions; (2) time-dependent boundary condi tions; (3) realistic magnetic field models; (4) tweaked, scaled, fit, activitydependent, or enhanced diffusion coefficients. This paper reviews these var ious modeling efforts, including the modifications that they required, their successes and failures, and lessons learned.