The objective of this study is to examine practical and cost constraints on where riblets, a new drag-reduction technology, might be used in internal flows. The broad categories of internal flow considered were HVAC ducting, gas pipelines, oil pipelines, water pipes, steam pipes, and pipes in refrigeration or air-conditioning machinery. Each category was first examined for practical constraints, which might complicate or exclude the use of riblets. Second, the potential power savings and the resulting cost effectiveness of adding riblets were estimated. For the more promising categories of internal flow, we then discuss what major questions must still be researched, and what technical developments might improve prospects.