Space Vehicles Directorate, Air Force Research Laboratory,
Hanscom AFB, MA, USA
I review the history of research on solar energetic particles (SEPs) beginning with the first report of a solar 'cosmic ray' event by S.E. Forbush in 1946. Since that discovery, SEP event taxonomy has evolved from a one-class picture in which particle acceleration was viewed as a 'delta-function' in space and time at the flare site to the modern view that SEPs are accelerated in a reconnection-based process associated with the flare and also at a temporally and spatially extended coronal/interplanetary shockwave. This evolution from a one-class to two-class picture was driven primarily by SEP composition and charge state measurements. More recently, such measurements -at higher energies than previously available- have opened the question of the source of > 25 MeV/nuc SEPs in large (gradual) SEP events. I conclude with a 'working hypothesis' classification scheme based on the shock-based work of Tylka and colleagues that appears to accommodate the new high-energy observations.