Arco demonstrated time-lapse 3D seismic, or “4D,” in the 1980s. The technology is focused on drilling better wells and improving reservoir management, thus increasing the poor 30–35% average reservoir recovery factor. It is used to extend the peak-flow plateau of the reservoir, deliver low-cost reserves additions, and make better use of topsides facilities. 4D is one of the few technologies that can show what is happening between wells. Its take-up was accelerated by the tendency of reservoir flow rates to come off-peak quickly and unexpectedly, requiring urgent improvements in the understanding of the reservoir. 4D technology led to improved well success rates and produced net-present-value (NPV) figures that were easily eight to 10 times the cost of the 4D seismic itself. It became commonplace to re-record 4D surveys at intervals of between two and five years. The technology has improved markedly since its early use, with nonrepeatability metrics falling from 40% or more down to single figures. It is most effective if conducted early in the reservoir's life and, subsequently, prior to reservoir management changes such as commencement of injection. On large reservoirs, 4D is generally routine, although it is also used on small reservoirs with 30 million bbl remaining reserves or less. Emplaced systems with frequent reshoots have been implemented on some reservoirs. Challenges include 4D on land, hard rock reservoirs, pressure response detection, and incorporation of geomechanical models. The 2015–2017 market downturn has slowed the extension of emplaced systems and other “broadband” techniques for 4D, but new technologies will move into this market, at reduced cost, so the future is full of opportunities.