The Pesticide Economic and Environmental Tradeoffs (PEET) decision-support system was designed to help farmers and managers select weed management practices while considering the impact of their choice upon economic return and groundwater quality. PEET estimates the economic return on different labeled herbicide treatments for the crop, soil, and weeds in the field of interest. This information may indicate that no weed control treatment will be economical so no pesticide needs to be applied. It also simulates the leaching and degradation of each herbicide in that soil and calculates a groundwater hazard index as the ratio of the estimated concentration of the active ingredient in the groundwater to a critical concentration for that chemical. The groundwater hazard is calculated using Monte Carlo techniques to deal with unknown future weather at the site of interest. By presenting both the economic return and the groundwater hazard, PEET enables a user to select a treatment after considering both the risk each product poses to groundwater and the economic benefit of each application. PEET was developed for use in any crop and geographic area. A database manager is available to enable specialists to enter and maintain data for crops and soils in new geographical areas. PEET is accessible on the internet. Clearly the calculated economic return and groundwater hazard depend upon the parameters entered by the PEET user and those stored in the supporting databases. The economic return for each potential herbicide treatment is uncertain due to uncertainty in yield, price of the product at market time, weed species present and their densities and sizes, and efficacies of each treatment for each weed species. The groundwater hazard index is uncertain due to spatial variability of soil properties, unknown future weather at a site, uncertain sorption and degradation rates, uncertain critical concentration values and uncertain aquifer characteristics. Users of the software deserve to know the uncertainties in the economic return and groundwater hazard for each chemical and ultimately the uncertainty in any decision based upon these values. Ideally, the absolute economic return and groundwater hazard values would be calculated accurately and reliably in spite of uncertain inputs. Recognizing that this may not be the case, it is also of interest to see if the choice of treatment is highly sensitive to the uncertainties. This research was carried out to quantify these uncertainties and to present them in forms appropriate to PEET users. The results of this analysis also provide insight into critical knowledge gaps requiring additional research. The sensitivity of the groundwater hazard is highly dependent upon the chemical. For cotton, 29% of the herbicide treatments labeled in Oklahoma would not exceed the critical concentration if 100% of the herbicide applied reaches the aquifer and mixes in a 1 m depth. For peanuts, 9% of the treatments would not exceed the critical concentration. The risk posed by these chemicals will be very insensitive to most uncertainties considered. Analysis of the model indicates that the relative uncertainty in groundwater hazard is equal to the negative relative uncertainty in critical concentration, mixing depth, or aquifer porosity. For treatments that pose a significant risk to groundwater quality, soil organic carbon and the soil depth below which degradation is assumed to be negligible are very critical values. When considering the groundwater hazard of treatments relative to other treatments, aquifer mixing depth and aquifer porosity do not impact the relative ranking of the treatments. Uncertainties in other parameters continue to provide uncertainty in the rank of some but not all treatments. Additional output options in the PEET software are under development to effectively communicate these uncertainties to the end user.