For many Shell-operated fields around the world, time-lapse reservoir monitoring (4D) is now an integral part of field management and some 25 dedicated 4D surveys were acquired for this purpose by the end of 2002. This widespread application is the result of a focused implementation effort aimed at global deployment to maximize the value extracted from the surveys in terms of saved costs, increased production, increased recovery and improved HSE management, where effective implementation is achieved through a combination of global operatorship and technology capability. Apart from ensuring global deployment, there is the challenge to extend the range of 4D applicability. To achieve this the project has three application portfolios: today's‘ proven’ 4D technology portfolio, which is about monitoring fluid movements in thick clastic oil reservoirs offshore. Results come largely from comparing reservoir simulator output with difference maps derived from repeat 4D streamer surveys; a‘ stretch’ portfolio where the technology is applied to gas reservoirs, land data, stacked reservoirs, carbonate fields and to the monitoring of pressure changes; a‘ tomorrow's technology’ portfolio, which has the potential to increase the application base even further. The new technologies are about the use of permanent arrays, downhole acquisition, passive listening and ‘smart fields’ where semi-continuous 4D monitoring provides eyes and ears between the wells. More and more value is realized as 4D becomes fully integrated with subsurface work flows and modelling tools. Benefits from 4D technology for individual fields can be in the tens or hundreds of millions of dollars. A large fraction of these come from 4D surprises, illustrating that we tend to underestimate our uncertainties and suggesting different approaches to field management.