A database was compiled containing index properties, compaction data, and hydraulic conductivity data from 51 clay liners. The hydraulic conductivity data included large-scale field measurements (KF) and small-scale laboratory measurements (KL) on specimens collected from the field in 74-mm-diameter sampling tubes. Analysis of the database shows that KF is comparable to KL when compaction is wet of the line of optimums, but KF can be up to 266 times larger than KL when compaction is dry of the line of optimums. Similar results were obtained from a stochastic simulation model. Discrepancies between KF and KL exist when macropores are present (i.e., compaction dry of the line of optimums), because macropores are inadequately represented in small laboratory specimens. To achieve, KF ∼ KL compaction specifications should require that the percentage of data falling wet of the line of optimums (Po) exceed 80–90% and the difference in initial saturation (ΔSi, which is the mean degree of saturation at compaction less the degree of saturation corresponding to the line of optimums) exceed 2 to 4%. A “line of optimums” compaction specification is recommended to facilitate achieving these recommended values for Po and ΔSi.