Low pT phenomena observed in high energy nuclear collisions

For several decades, experiments have reported an enhancement in the pion invariant cross section at low pT (<300 MeV/c) when compared to a thermal fit or minimum bias pp results. Experimental results from pp reactions at the ISR to heavy ion collisions at CERN will be reviewed then compared to extract trends. Several theoretical explanations will be discussed. An emphasis will be placed on the role of resonances and a comparison of data to models will be presented. (orig.)