The most common radon system is a sub-slab depressurization system.  A fan, located in an attic or outside the building, is used to draw air out from under a basement, crawl space or slab on grade concrete slab.  If done properly, the entire area directly below the slab becomes negative in pressure as compared to the air above the slab.  This causes the normal airflow out of the soil to reverse and flow into the soil, which effectively stops all infiltration of radon laden soil gas.  The sub-slab depressurization system needs to create this sub-slab negative pressure under all slabs that are contributing a significant amount of radon into the building.