Time-lapse (4D) seismic is a key component of an asset's reservoir surveillance strategy. Its value lies in optimizing field developments because it enables the detection of reservoir changes due to production, and it allows for timely corrective action toward optimal field management. In producing assets, 4D seismic adds incremental economic value by providing 3D information on the dynamic performance of reservoirs. This can occur during each of the four phases of the life of a field: first, by helping extend the field's base production and injection plateau through the identification of infill target areas (bypassed hydrocarbons) or areas of inefficient injection support; second, by adjusting depletion plans and optimizing hydrocarbon recovery mechanisms and their efficiency; third, by rejuvenating the field through taking advantage of existing infrastructure to develop stranded pools and targeting poorly swept areas of the field; and fourth, by managing the life of field more effectively through monitoring saturation and pressure changes at both reservoir and well scales. The value gained from deploying 4D seismic often exceeds our expectations when compared to that derived from a value-of-information exercise prior to acquiring such data. This is due to the “known unknowns,” such as flow features that occur below the seismic scale, and “unknown unknowns,” such as complex reservoir connectivity. We present several field examples and show that the source of value falls under two main categories: quantifiable, such as net present value (NPV), and qualitative, such as improved field knowledge and decision making. We also discuss a case where 4D gave rise to an incremental increase in NPV that exceeded one-third of the field's total original NPV.