Microfluidic systems offer the promise of integrating multiple laboratory processes ranging from sample injection, filtration, pumping, mixing, separation to detection within a single chip, thereby increasing the throughput and decreasing the costs. Further, the interconnected networks of microchannels and reservoirs of tiny volumes of samples well match the demands for rapid response, massive parallel analyses, automation, and minimal cross-contamination, as required in many practical applications, especially those dealing with the analysis of chemical, biological, or biochemical materials. A rapid and efficient mixing of these substances to be handled is often considered as an important determinant that contributes to an appropriate functioning of the microfluidic systems. Because low Reynolds numbers are typically met for flows in microchannels, the benefits of turbulence for enhancing the mixing process are not obtained. In view of this, the efficient mixing of samples in microfluidic devices remains a bottleneck hindering advancements of fully integrated microfluidic systems. In this chapter, particular attention is paid to a comprehensive description of various micromixers that allow for an efficient mixing of fluids. The latter may be achieved according to two radically different processes. In one of these, the mixing process relies on the diffusion and/or chaotic advection of the pertinent molecules without the necessity of using external actuators except for fluid delivery. Those mixers are thus qualified as passive. In the second type of process called active, mixing is achieved by a judicious use of energy input such as that provided by externally applied electric or magnetic fields. The functioning, advantages, drawbacks, and respective performances of these various types of mixers corresponding to one or the other process are described in detail.