  Single spin asymmetries in polarized proton(antiproton) + proton -> pion + X
have been measured to be large. They can be understood within perturbative QCD
by introducing new k_perp and spin dependences in fragmentation and/or
distribution functions. We discuss first how these effects in distribution
functions alone can account for existing data and predict single spin
asymmetries in other processes; then we see how similar effects in
fragmentation functions alone can fit the existing data; finally we devise
strategies to discriminate between the two contributions
