Following experimental success in the application of 4D technology in the Magnus and Foinaven fields and the delivery of commercial value from the application of time-lapse seismic (4D) studies in the Forties and West of Shetland fields and in the Pompano Field in the Gulf of Mexico, BP has embarked on the systematic use of 4D monitoring in its NW Europe operations to enhance value in the light of an increasing production challenge. Within the past two years, 12 datasets have been acquired over 19 fields of varied reservoir age, type and property. Although specific questions differ from field to field, in all cases a major objective is to influence the location and design of infill wells. Useful time-lapse information has been gained from post-stack matched "legacy" surveys of varying data quality. However, it is clear that more value is added by custom-designed 4D surveys: seismic quality and repeatability are critical to the successful delivery of reliable time-lapse seismic monitoring. Integration of seismic data with dynamic data is still relatively "low-tech" and further benefits will be gained by improvement in this area. The paradigm is shifting to a position where 4D monitoring in the North Sea is the rule rather than the exception. Now that there is clear evidence of added value in the most obvious target reservoirs, surveys are being carried out in areas that are technically more challenging and the early results indicate that these too will be of real commercial benefit. Use of 4D data is also being progressed in other regions, including the Gulf of Mexico, and application in other basins around the world is being planned. On the technology development front, BP is seeking continuous improvements in acquisition and processing, both in quality and turnaround time, and richer integration with production and conventional well-based surveillance data. The ultimate goal is for effective reservoir management based on a range of surveillance methods.