The injection of seawater into oil-bearing reservoirs to maintain reservoir pressure and improve secondary recovery is a well-established, mature operation. Moreover, the degree of risk posed by deposition of mineral scales to the injection and production wells during such operations has been much studied. The current deepwater subsea developments offshore West Africa and Brazil have brought into focus the need to manage scale in an effective way. To this end, the challenge of scale control during the lifecycle of water injection, production, and produced-water reinjection (PWRI) has been reviewed for a number of fields by the authors. This paper outlines the risk assessment process that should be undertaken to select the most economical and effective scale control methodology (which for sulfate-based scale could be seawater injection with scale inhibitor squeeze treatments to maintain production, or sulfate reduction of the injection water with or without the need to scale inhibitor squeeze). In the case of sulfate reduction, parameters to be investigated include the degree of desulfation required to minimize the scale risk of downhole scale formation, the impact the degree of fluid mixing will have on the resulting brine (from injection to production), and the impact the desulfated brine will have on scale control during PWRI. The paper draws on a wide range of technical inputs to make scale management decisions including: computer modeling techniques (e.g., deposition models that incorporate the kinetics of sulfate scale formation at low supersaturation ratios), reservoir simulation of fluid mixing and reaction; the resulting produced brine chemistry, laboratory-generated coreflood data to assess chemical selection for scale inhibitor squeeze and produced water application; and field results that will demonstrate the impact of the type of injection water source on the long-term manageability of such deepwater projects. Finally, the paper outlines in detail the particular issues associated with the full economic assessment of low-sulfate water injection vs. full sulfate seawater injection.