Chemical separations are a critical component of analytical and synthetic chemistry. In microchip applications, a sample comprising multiple chemical species is separated spatially into individual components by inducing the components of a sample to move at differing velocities in a microchannel. This is shown schematically in Fig. 12.1 and a sample experimental result is shown in Fig. 12.2. Separations are achieved by inserting a sample fluid bolus into a microchannel, inducing motion of these species with velocities that differ from species to species, and detecting the concentration of species as a function of time as these species elute (i.e., arrive) at the location of the detector (Fig. 12.1). Many microfluidic separations are modified from capillary or column-based techniques, and draw advantages from more optimal fluid transport, thermal dissipation, or system integration. One example of a chemical separation is an electrophoresis separation, which can be used to separate species that have different electrophoretic mobilities. In this case, species motion is induced by an electric field aligned along the axis of the microchannel, which induces electroosmosis and electrophoresis. Because this technique requires only that electric fields be applied, it integrates easily into microsystem designs, and a large fraction of the microchip analyses developed since 1995 use microchip electrophoresis (see one example in Fig. 12.3). This is true for both protein analysis (Section 12.5) and DNA analysis and sequencing (Chapter 14).