2010 and future EPA regulations introduce stringent Oxides of Nitrogen (NOx) reduction targets for diesel engines. Selective Catalytic Reduction (SCR) of NOx by Urea over catalyst has become one of the main solutions to achieve these aggressive reductions. As such, urea solution is injected into the exhaust gas, evaporated and decomposed to ammonia via mixing with the hot exhaust gas before passing through an SCR catalyst. Urea mixers, in this regard, are crucial to ensure successful evaporation and mixing since its liquid state poses significant barriers, especially at low temperature conditions that incur undesired deposits. Intensive efforts have been taken toward developing such urea mixers, and multiple criteria have been derived for them, mainly including NOx reduction efficiency and uniformity. In addition, mixers must also satisfy other requirements such as low pressure drop penalty, mechanical strength, material integrity, low cost, and manufacturability. All these criteria must be fully integrated into the development process with the assistance of predictive tools, emission tests, material selection, durability tests, and manufacturing feasibility studies. This paper summarizes the development status of multiple mixers that have demonstrated excellent overall performance, as well as testing and CFD methodologies to compare benefits and quantify performance.