Tailings dams differ from conventional earthfill structures in that much more time is available to optimize design since their construction is extended over a longer period. Laboratory data are presented on a variety of tailings sands which indicate that they are relatively strong and incompressible. Poor performance is likely to result from inadequate seepage control and instability due to liquefaction. In order to take advantage of opportunities for ongoing design,in situpermeability and density monitoring is needed together with pore pressure measurements. An infiltration test for the determination of permeability above the water table is proposed.In situdensity determinations are made by driving a nuclear probe into the sands. The latter technique offers considerable promise for evaluating liquefaction potential of natural and fill deposits. Field experience is used to illustrate the application of the techniques.