What have we learned and want to learn from heavy ion collisions at CERN SPS?

The talk is a mini-review of current status of the field, with emphasis on SPS heavy ion program, now and beyond 2000 (as asked by the organizers). The main question is, of course, whether we can convince ourselves and community at large that the QGP is in fact produced at SPS. We came a long way toward the positive answer, and are definitely on strongly rising part of the learning curve. Still, in few key directions we lack important pieces of evidences.