Forest managers have a standard set of tools they use to reduce fire hazard: mechanical thinning, brush clearing, mechanical treatment of slash (small woody debris), prescribed fire, and various combinations and timings of the use of these tools. Although these tools are widely used, the science is sketchy on the benefits and tradeoffs of the different treatments. In response, the national Fire and Fire Surrogates Study (FFS) set up a national network of research sites to study the effects of fire 'surrogates,' such as mechanical thinning, mechanical slash treatments, and prescribed fire on forests. Early findings for the Sierra Nevada FFS site are reported here. The study used four treatments: prescribed fire-only, mechanical-only, mechanical-plus-fire, and no-treatment controls. All three active fuel treatments significantly reduced fire risk, but the two treatments that used prescribed fire to reduce surface fuels achieved the greatest reductions in potential fire behavior. The mechanical-only treatment (mechanical thinning followed by mechanical slash treatment) reduced crown bulk density and ladder fuels but increased surface fuels, and it was less effective in reducing fire risk. The active treatments also had noticeably different consequences on forest structure and predicted tree mortality. A pretreatment assessment can determine the level of fire hazard from the surface, ladder, and crown fuels, and a prescription can be designed to treat the fuel layers creating the risk.