In order to make critical loads most useful to policy makers and resource managers, it is essential to quantify the uncertainty associated with the critical load. In the past, a rating system developed in Europe which uses a 3 point scale: ##-reliable, #fairly reliable, (#) expert judgment was most often used for critical loads in the US. The new system that we have developed uses a 5 point scale to allow finer nuances in reliability of the critical loads. The factors that affect the certainty of the critical load include the number of sites, the number of samples, the representativeness of sites to the region being evaluated, and the strength or clarity of the response on which the critical load is based. We have created two tables detailing the criteria that define each of the rating levels from the most uncertain (1) to the most robust (5). Different combinations of criteria can lead to the same rating (for example, a large sample size with a moderately strong response could be in the same category as a strong response with a moderate sample size). The first table is for heterogeneous data, that is, data from many different studies that are combined for the purpose of estimating a critical load; the second table is for large datasets of homogeneous data (e.g., FIA data). This uncertainty framework is being incorporated into several national-scale critical loads projects and should improve the ability to compare critical loads and better understand their implications.