Meeting future food demand without massive expansion of crop area at expense of natural habitat will require future farmers to achieve yields much closer to the yield potential ceiling on land suitable for intensive production. Scientifically sound, reproducible, transparent methods to estimate crop yield gaps (Yg), which are the difference between potential yields under irrigated (Yp) or rainfed (Yw) conditions and actual farm yields (Ya), are essential to inform polices and research priorities that seek to ensure national and global food security, and for improving efficiency of field research on sustainable intensification. In this presentation we review published literature on yield gap assessments from local to global scales, and use insights thus derived for recommendations on “best practice” methods. A key finding is that a “bottom-up” , location-specific approach is required for robust estimates of Yp or Yw based on crop simulation, which in turn requires a method for upscaling local Yg estimates to regional, national or global scales. A novel climate zonation (CZ) scheme has been developed to support upscaling of Yp, Yw, Yg and Ya at locations within CZs with greatest harvested area density for the crop in question. Location-specific estimates at these crop-dense sites require long-term weather data, and information on soil properties and key crop management practices (time of planting and crop maturity within the dominant existing cropping systems) at those locations. While acquisition of these data is not difficult in many data-rich developed countries, good quality data are often lacking in many developing countries. To overcome this challenge, the Global Yield Gap Atlas (GYGA, www.yieldgap.org) has developed a set of protocols for obtaining, or generating, required data for weather and crops and for calibrating crop simulation models needed to estimate Yp and Yw. Key to this process are a cadre of collaborating agronomists familiar with requirements for robust yield gap estimates and knowledgeable about agronomic practices used on major food crops in their country. The aspiration of the GYGA project and its team of collaborating agronomists is to estimate yield gaps for all major food crops on every hectare of existing farmland.