Micrometre-scale analytical devices are more attractive than their macroscale counterparts for various reasons. For example, they use smaller volumes of reagents and are therefore cheaper, quicker and less hazardous to use, and more environmentally appealing. Scaling laws compare the relative performance of a system as the dimensions of the system change, and can predict the operational success of miniaturized chemical separation, reaction and detection devices before they are fabricated. Some devices designed using basic principles of scaling are now commercially available, and opportunities for miniaturizing new and challenging analytical systems continue to arise.