Improved hydrocarbon recovery through the identification of unswept oil regions has been the primary objective of the time-lapse seismic programme at the Gullfaks Field in the North Sea. Three time-lapse seismic surveys covering the entire field have been acquired, a pre-production survey in 1985 followed by two surveys in 1996 and 1999. Input from qualitative time-lapse interpretations has resulted in the drilling of a number of successful wells, and has improved the understanding of drainage patterns. Recently, a quantitative time-lapse study was launched. The objective of the study was two-fold: to link flow simulation predictions directly to the 4D seismic response and to map changes in oil saturation and quantify associated uncertainties.