Publisher Summary This chapter discusses the indirect techniques and direct techniques of measuring the electroosmotic flow velocity. Electroosmotic pumping is a preferred method in a variety of lab-on-a-chip devices because the electroosmotic pump has no moving mechanical parts, has zero-volume, and provides uniform cross-section flow. To design and operate an electroosmotic pump, it is critical to know the volumetric flow rate or the velocity of the electroosmotically pumped flow. As the flow rates involved in the lab-chip devices usually are very small, and the size of the microchannels is very small, it is extremely difficult to directly measure the flow rate or velocity of the electroosmotic flow in these microchannels. The chapter explains that to study liquid flow in microchannels, various microflow visualization methods have evolved. Micro particle image velocimetry (microPIV) is a method that was adapted from well-developed PIV techniques for flows in macro-sized systems. In electrokinetic flows, the electrophoretic motion of the tracer particles is an additional consideration that must be taken. These are the disadvantages of the microPIV technique. Dye-based microflow visualization methods have also evolved from their macro-sized counterparts. However, traditional mechanical dye injection techniques are difficult to apply to the microchannel flow systems. In additional to these PIV and dye-based techniques, the electroosmotic flow velocity can be estimated indirectly by monitoring the electrical current change, while one solution is replaced by another similar solution during electroosmotic flow.