fax 01-972-952-9435. AbstractMaintaining zonal isolation for the lifetime of oil and gas wells is critical. Leakage behind casing can reduce the costeffectiveness of the well and cause health and safety risks from pressure build-up and contaminated aquifers. During the completion and production phases of the well, temperature and pressure variations can cause stresses at the cement-toformation interface. The ability of the casing-cement system to maintain a seal at the cement-to-formation interface depends on the condition of the formation surface prior to slurry placement. The condition of a shale will depend upon the nature of the drilling fluid used, whereas the condition of a permeable rock will depend upon the presence and nature of the filtercake deposited during drilling and circulation.In this paper we present an improved understanding of chemical interactions at the cement-to-formation interface and the factors that determine bond strength and the position of the plane of failure. For permeable formations, the role of the mud filtercake for different mud types is explored. For nonpermeable formations, the presence and effect of a residual mud film are also examined. The extent and depth to which chemical alteration of the mudcake occurs when in contact with cement are determined, together with measurements of the yield stress and water content profile of the altered mudcake. The effect of exposing swelling and nonswelling shales to inhibitive drilling fluids on bond strength is presented. Laboratory-scale test equipment and a small-scale wellbore simulator, developed for tests under realistic field conditions, are described. The flexibility of cement plays a role in bonding and is demonstrated by the simulator tests. This improved understanding allows us to confirm the key issues at the cement-to-formation interface and propose some solutions for effective zonal isolation.