Abstract The Brent Field commenced production in 1976, with annual oil production peaking in 1984 at 410 Mbbl/d. Since the mid 1980s, oil production has been declining, but because of the high solution GOR substantial gas reserves remain, dissolved in the residual and bypassed oil. In 1992 the decision was taken to depressurize the Brent Field to recover an additional 1.5 Tscf of gas and 34 MMstb of oil, extending the field's life by 5-10 years. This would entail redevelopment of three of the four Brent platforms at a total cost of £1.3 billion to install process facilities for low-pressure operations, to reduce operating costs, to implement safety upgrades, and to refurbish facilities. The fourth platform was also upgraded but no low-pressure facilities were installed. Preparation of the platforms for long-term field development was completed at the end of 1997. This paper addresses the processes and systems to measure reservoir performance against plan as depressurization of the Brent Field gets underway. By several measures, 1998 has been a year of significant change. The number of well entries, gas export, gross liquid production, and reservoir pressure change were all at their highest ever levels, whilst the field's oil production remained significant in U.K. terms. Implementing the reservoir management strategy requires striking an appropriate balance between well activity which provides an immediate return on oil or gas production and well activity which is to produce mainly water in order to liberate gas for later benefit. Essential to the decision making process is the acquisition of surveillance data, of sufficient quality and coverage, to allow proper tracking of the rapidly changing field conditions. This surveillance data then needs to be translated quickly into meaningful information, such as graphical displays of oil/gas/water distribution and quantification of parameters such as free gas cap size. This information is compared with previous predictions, with the history-match being revised where necessary, resulting in continued refinement of the static and dynamic reservoir models. Accordingly, depressurization performance can be monitored against plan and variances recognised early, leading to the optimum response in production or investment profile.