Microfluidic devices are becoming more common and essential to a range of technologies. In their Perspective, [Ottino and Wiggins][1] discuss recent results in an important aspect of this field: micromixing. In contrast to everyday fluids, fluids flowing in small structures are dominated by viscous effects. This in turn makes mixing difficult. One approach for designing optimal mixing devices comes from the world of pure mathematics. In particular, a tool called a "linked twist map" offers the possibility of predicting which structures and designs make the best mixers. [1]: http://www.sciencemag.org/cgi/content/full/305/5683/485