Proprietary underground devices have become a popular means of stormwater treatment in dense urban areas due to tight space constraints. Several technologies are employed to remove sediment and other debris from stormwater runoff prior to discharge into lakes, rivers, and streams, including hydrodynamic separation and filtration. Field monitoring studies have been undertaken which have shown these devices, especially the hydrodynamic separation systems, to be capable of removing a significant amount of solids, or incapable of removing solids contained in surface runoff. The results of field monitoring studies depend upon accurate sampling of the runoff, both upstream and downstream of the device. Obtaining representative samples of suspended sediment at each location is a challenge. This research explores two objectives: 1) the feasibility of controlled field testing, and 2) the performance of each device when subjected to field testing under a variety of treatment discharges, and with a wide range of sediment sizes. Each device is carefully cleaned prior to the testing and then a bulk solids analysis is performed on sediment captured by the device so as to eliminate errors due to sampling of influent and effluent water. After data analysis is complete, a revised sizing criterion is proposed which will improve overall performance and sizing of such devices. The resulting approach, refined through field experiments, will be incorporated into an assessment (monitoring and field testing) protocol that will be used by consultants, local governments, and state agencies to assist in selecting, designing, and evaluating stormwater treatment technologies for public infrastructure improvement projects.