As the need to retrofit secondary treatment plants for biological nutrient removal (BNR) has become widespread, process variations such as step-feed BNR have been developed to adapt existing treatment facilities to meet effluent requirements. Since there are relatively few fullscale installations of step-feed BNR, the process and retrofit design concepts are still under development. Although step-feed BNR has a relatively small footprint for a suspended growth process, it also is more complex than plug-flow or complete-mix regimes, and presents unique challenges to the designer. This paper discusses major design components including primary effluent flow splitting, chemical feed, dissolved oxygen carryover, nitrate recycle, and foam control; illustrates their impact on performance through process modeling examples; and shows how they are being resolved in New York, NY; Cumberland, MD; and North River, VA.