Slurry cut-off walls are non-structural walls constructed underground to intercept and impede groundwater flow. Principal applications are site dewatering, pollution control, and seepage barriers in the foundations of water retaining structures. Two basic types of trench — soil-bentonite and cement-bentonite, and the principal kinds of slurry trenching equipment are discussed. Examples of several recent projects are given. Slurry cut-off walls normally have a permeability in the range of 10^-6 to 10^-7 cm/sec. The primary purposes of quality control are to check the continuity and depth of the wall, and to ensure a slurry and backfill which falls within workable limits while satisfying design criteria. Temporary variations from optimum limits are permissible; later batches can be used to correct the values, and the digging action will mix the slurry to create a relatively homogeneous mixture. In the attached Appendix, a set of Annotated Specifications for soil-bentonite and cement-bentonite trenches is presented.