Paper is a centuries old substrate for carrying out chemical measurements. The application of paper as a platform for microfluidics, however, was only introduced to the scientific community less than ten years ago and has become a major new field of research. Microfluidic paper-based analytical devices are attractive as sensors and analytical tools because they are very inexpensive (often with substrate costs of <$0.01US), generate flow via capillary action in a controlled and predictable manner, have the ability to store reagents, and can achieve low detection limits for a range of detection methods. This review covers developments made over the last two years in this rapidly expanding field. We first discuss basic processes affecting water wetting and flow from a fundamental predictive basis. We then discuss developments in fabrication methods and device functionality before covering detection methods. Finally, we summarize papers addressing new applications of paper-based analytical devices.