Through a combination of computational fluid dynamics and rapid prototyping, we have developed various designs of mixers for millifluidic and microfluidic applications. Two designs will be presented: a mixer based on the Dean vortex, and a groove-based (surface-patterned) mixer. The Dean-based mixer has a low surface area-to-volume ratio, reducing the risk of fouling, but requires a high flow rate to operate. The groove-based mixer has a higher surface area, but operates over a wide range of flow conditions. In the process of developing these mixers, we have gained a deeper understanding of how to control the movement of the fluid streams within the channel. We are developing designs that will improve mixing in shorter path lengths.