Microfluidics is a rapidly growing field being driven by new technological applications in the medical, materials, and chemical sciences. For example, developing biotechnologies require handling small quantities of many different fluids. With the increasing complexity of experiments, devices have shrunk, and the strategy of “smaller is better” has begun to transform the world of fluidics as it has transformed the world of electronics. As the scale on which we study fluids decreases, the effects driving and dominating fluid motion change radically. At micron scales, fluids are primarily dominated by surface tension and viscous forces. Naively, one might think this simplifies the analysis of fluid motion. However, a new set of physical effects begin to become relevant on these scales. Gradients in surface tension, due to temperature or solutal variation yield Marangoni stresses. Intermolecular forces (in particular van der Waals forces) between fluids and substrates can drive the rupture of thin films. Surfactants can lead to dipolar surface charge distributions that can drive surface motion. Moreover, at these scales the continuum approximation can begin to fail; ordering of molecular orientation on a solid surface can lead to additional stresses. Surface roughness and inhomogeneities neglected at terrestrial scales must also be modeled to faithfully reproduce experimental results.