Throughout the Institute's history, NIOSH has engaged in occupational safety and health research in extraction industries (e.g., mining), but only since 2005 has NIOSH focused on upstream oil and gas extraction. During initial site visits at oil and gas wells, it was observed that notable amounts of silica-containing proppant, or frac-sand, are used during hydraulic fracturing operations. This observation led to research of workers' exposures to silica at 11 sites in 5 states. In 2010-2011, full shift (typically 12 hours) personal breathing zone samples for respirable silica were collected from workers in 15 job categories who participated voluntarily. Sample analyses were performed by an accredited laboratory, using validated NIOSH methods. Time-weighted average concentrations of respirable crystalline silica were calculated for comparisons with the Occupational Safety and Health Administration permissible exposure limit (PEL), the NIOSH recommended exposure limit (REL), and the American Conference of Governmental Industrial Hygienists threshold limit value (TLV®). Of a total of 111 individual samples collected, 93 (83.8%) exceeded the TLV, 76 (68.5%) exceeded the REL, and 57 (51.4%) exceeded a calculated PEL, in some cases by more than 10-fold the PEL, REL, or TLV. By job category, sand mover and transport belt operators had the highest exposures. Risks for silica exposures are significant during hydraulic fracturing due to the quantity of sand used and the way it is handled. Seven primary points of dust generation were identified in this study along with recommendations for controls.