NH3 is readily released into the air from biological, combustion, waste disposal, and industrial sources. It is estimated that 3.9 Tg/yr of NH3 are emitted into the atmosphere in the US. The National Atmospheric Deposition Program/National Trends Network (NADP/NTN) has monitored wet deposition of ammonium and other major ions across the US since 1978. The NADP's Ammonia Monitoring Network (AMoN) was established in October 2007 to routinely measure NH3 using cost-efficient passive type air samplers. AMoN provides land managers, air quality modelers, ecologists, and policymakers critical data to assess compliance with PM2.5 standards, among other objectives. The completion of 2012 measurement data provides 5 yr of data for the NADP/AMoN. Long-term (>5 yr) trends in ambient NH3 levels across the US were analyzed based on AMoN measurements. The NADP/AMoN provides consistent data of sufficient quality to determine long-term trends in ambient NH3 across the US. Statistically significant and seasonally homogeneous trends were determined at 5 of 14 sites evaluated that had a minimum of 5 yr valid data. All significant trends were increasing, although the overall magnitude was small in comparison with mean annual concentrations (1%/yr). This is an abstract of a paper presented at the 106th AWMA Annual Conference and Exhibition (Chicago, IL 6/25-28/2013).