This may be one of the most misunderstood building components and is one of the discussions I hear most often.  The misconception is that poly under a concrete floor will stop moisture from showing on the surface.  The poly, aka plastic, creates a barrier from moisture being introduced from the bottom side of the floor which helps in overall moisture content; however, most of the complaints come at the time where the floor is much colder than the warm, moist air above it, commonly is the spring, which is the formula for condensation.  Poly under the slab does nothing to correct this situation.