Very large solar energetic particle (SEP) events occur at the rate of about 10 per year during solar maximum. The primary accelerators in these events are shocks driven by fast coronal mass ejections, in which speeds are greater than about 1000 km/s. Solar Cycle 23 probably produced the largest fluence of > 10 MeV/nucleon SEPs seen since the start of the Space Age. New instruments on Wind, ACE, SOHO, and other satellites have provided unprecedented detail on the energy spectra, elemental and isotopic composition, ionic charge states, and temporal evolution of these SEP events, as well as their associated CMEs and flares. In this talk, I will review some of the new insights provided by these data. A particular challenge in SEP studies has been the very large event-to-event variability in composition and spectral characteristics, particularly at energies above a few tens of MeV per nucleon. I will discuss recent efforts to understand this variability in terms of seed populations and shock geometry. I will also review recent studies of the time at which SEPs first appear on the Sun-Earth magnetic field line and the implications of these studies for the conditions under which SEP production was initiated.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html