Cut-off walls built by the bentonite-clay slurry trench technique are designed and installed to control flow during site remediation at hazardous waste sites. Non-aqueous phase liquids (NAPL's) in the site groundwater are being sought in remedial investigations as critical contaminants which are likely to significantly affect cut-off wall design and performance. Testing of backfill permeability with these liquids is necessary to ensure that the backfill will resist degradation at elevations where they occur in the groundwater. Recent research has focused on the impact of ion diffusion through the completed barrier. Plastic concrete, attapulgite clay slurries, and biopolymer slurries have been reported in use to meet job-specific requirements. Field test cells, several tens of feet in dimension, and extending to the expected depth of the total cut-off all structure, are increasingly being required by clients as pre-construction hydraulic performance demonstrations. Pump-down measurements applied to these cells provide data from which the permeability of the walls, built with full-scale field equipment, can be calculated. Hydraulic fracturing, verification of bottom-key effectiveness, long-term permeability performance, and significance of diffusion of solutes through the completed groundwater barrier remain areas of sparse published data. This paper reviews environmental slurry wall experience since the early 1980's, with emphasis on performance evaluation.