Recovery factors for comparable reservoirs are often from 10 to 20% higher in the USA than elsewhere. The reason is the relatively low cost of drilling and work-over combined with the high revenue per barrel for the producer in the States. This implies that elsewhere the scope for additional recovery will frequently be of the same order as the noted differences. Another motivation for re-evaluation of older fields is the realization that irreducible oil saturations measured on core plugs in the past were generally much too high. Consequently, the assumed sweep efficiencies were much too high also, resulting in little incentive for infill drilling or recompletion. A number of factors stimulate the re-evaluation of remaining reserves. Reliable through-casing logging makes the poorly swept zones visible. Detailed structural definition is often possible with 3D seismic. New drilling techniques, like short radius horizontal side tracks, allow economic recompletions of old wells.