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The behaviour of geosynthetic clay liners (GCLs) as part of a physical-environmental sys-tem is examined. Consideration is given to: (a) both the physical and hydraulic interactions with the mate-rials, and the chemical interactions with the fluids, above and below the liner, (b) time dependent changes in the materials, (c) heat generated from the material to be contained, as well as (d) the climatic condi-tions both during construction and during service. This paper explores some common perceptions about GCL behaviour and then examines the misconceptions that can arise and their implications. It demon-strates how what may first appear obvious is not always as one expects and that more is not always better. It discusses: (i) the pore structure of a GCL, (ii) the dependency of the water retention curve of the GCL on its structure, bentonite particle sizes and applied stress, (iii) the effect of the subgrade pore water chemistry, (iv) the mineralogy of the subgrade, and (v) thermal effects. The desirability of a GCL being reasonably well-hydrated before being permeated is examined. The critical size of needle-punch bundles at which preferential flow can increase hydraulic conductivity by orders of magnitude is illustrated. The dependency of self-healing of holes on the interaction between GCL and subgrade is discussed. Finally, the transmissivity of the geomembrane/GCL interface is shown to be a function of GCL and geomem-brane characteristics and to be poorly correlated with GCL hydraulic conductivity.