Glenn Mason
   University of Maryland, College Park, Maryland USA

Corotating interaction regions (CIRs) arising out of the interaction of fast- and slow-solar wind streams provide excellent opportunities for studying shock acceleration in the heliosphere. Studies in the 1970s established that the ion composition in these regions was similar to that seen in solar energetic particles, with significant differences for elements such as helium and carbon. New instruments recently launched on Wind and ACE have extended observations of CIRs to much lower energies than before, providing new insights into the composition and spectra, and establishing stringent constraints on acceleration and transport models. Taken together with pick-up ion studies, and solar wind composition provided by instruments on Ulysses, Wind, and ACE, it is now possible to address issues of CIR seed population and injection based on direct observations. We will summarize the current observational status and discuss its implications for energetic particle acceleration and transport.