The performance of clayey liners and covers is assessed by several methods including borehole variable-head permeability (k) tests. Different interpretations of the end-of-casing and Lefranc tests are discussed. In Quebec, most tests have been interpreted using the velocity-graph method. Its equations are presented and differences are illustrated by examples. Numerical simulations confirm the validity of the velocity-graph method. It eliminates a frequent error in thekvalue resulting from a poor evaluation of the piezometric level for the test. Using an incorrect interpretation method may yield akvalue up to 50 times lower than the actual value. Smearing the clayey surface in the injection zone may be a source of uncertainty in thekvalue. A numerical analysis shows that a 1 or 2 cm smeared zone may change the testkvalue by over one order of magnitude. It is also shown that it is illusory to try to extract the anisotropy in hydraulic conductivity of the compacted clay.Key words: clay liners, clay covers, permeability, in situ test, interpretation.