Microfabricated fluidic systems allow complex chemical analyses to be performed on sub-nanoliter volumes of sample. Compared to macroscopic systems, these devices offer many advantages, including the promise of performing some analytical functions more rapidly and on smaller samples. However, miniaturization of analytic instruments is not simply a matter of reducing their size. At small scales, different effects become more prominent, rendering some processes inefficient and others useless. The small scales also permit the creation of novel devices, such as the H- filter, which we are using to extract analytes from whole blood. Fluid flow in microfluidic systems is entirely dominated by viscous forces, making diffusion the sole mechanism of mixing. In addition, a larger fraction of molecules are lost to surface adsorption as devices shrink. This paper examines some of the issues involved in device miniaturization, specifically those phenomena that become increasingly dominant.