A conceptual project aimed at understanding the fundamental design considerations concerning the implementation of catalyst systems on outboard marine engines was carried out by Mercury Marine, with the support of the California Air Resources Board. In order to keep a reasonable project scope, only electronic fuel injected four-stroke outboards were considered. While they represent a significant portion of the total number of outboard engines sold in the United States, carbureted four-strokes and direct injected two-strokes pose their own sets of design constraints and were considered to be outside the scope of this study.Recently, three-way catalyst based exhaust emissions aftertreatment systems have been introduced into series production on sterndrive and inboard marine spark ignition engines in North America. The integration of catalyst systems on outboards is much more challenging than on these other marine propulsion alternatives. Sterndrive and inboard engines are horizontal crankshaft engine derivatives of automotive products. Outboards on the other hand utilize a vertical crankshaft, open cooling systems, and consist almost entirely of components that were specifically designed for a marine outboard engine application.This report will show how Mercury Marine used state of the art processes and design analysis tools to successfully design a catalyst system for a production based outboard engine targeting combined hydrocarbon and oxides of nitrogen emissions performance equivalent to the sterndrive and inboard standard of 5 grams per kilowatt-hour over the marine engine emissions test cycle. Prototypes of the design were constructed and tested. Results of that testing will be shown that highlight the potential to meet future emissions requirements and some of the challenges that will face commercializing this technology.