Are parton distributions positive?

We show that the naive positivity conditions on polarized parton distributi ons which follow from their probabilistic interpretation in the naive parto n model are reproduced in perturbative QCD at the leading log level if the quark and gluon distribution are defined in terms of physical processes. We show how these conditions are modified at the next-to-leading level, and d iscuss their phenomenological implications, in particular in view of the de termination of the polarized gluon distribution.