Vertical barrier walls are routinely used to contain contaminated groundwater and vapors around old landfills and remediation sites. Three methods are available for evaluating the hydraulic conductivity of vertical barriers: (1) laboratory tests on reconstituted samples; (2) laboratory tests on 'undisturbed' samples; and (3) in situ tests. Test procedures and relative advantages of each method are summarized. Typical test results are provided for three projects. Different methods for measuring hydraulic conductivity can produce hydraulic conductivity's that vary by orders of magnitude, and not always in the same direction on different projects. Numerical analyses were performed for in situ slug tests, and comparative results are presented to clarify the significance of critical variables for single-well borehole tests on vertical barrier walls.