The report gives results of testing of indoor radon reduction techniques in 19 existing houses in Maryland. The focus was on passive measures: various passive soil depressurization methods, where natural wind and temperature effects are utilized to develop suction in the system; and sealing of radon entry routes into the house. Active (fan-assisted) soil depressurization techniques were also tested. Passive soil depressurization systems typically gave moderate radon reductions (30-70%), although the reductions ranged from zero to 90%. Only two houses were reduced <4 pCi/L with the passive systems. A passive system is most likely to be successful when sub-slab communication is very good, when the house has a basement with no adjoining slab-on-grade or crawl-space wings, and when the foundation walls are poured concrete instead of hollow block. Entry route sealing as a stand-alone radon mitigation measure gave zero-50% reduction in the only house where it was tested. Active soil depressurization, tested in 18 houses, reduced 16 of them <4 pCi/L, and 12 of them <2 pCi/L; reductions were often >90%. Poor sub-slab communication prevented this approach from being fully successful in the other two houses; later modifications to these two systems reduced these houses <4 pCi/L also.