In microchannels with typical dimensions from 10 m to a few hundreds of micrometers, the flow is dominated by viscous forces, often leading to laminar flow conditions. At the entrance or in bends and curves, where the flow changes its velocity or direction, inertial forces generate transverse flow velocities. Due to continuity, vortex pairs, such as Dean flow in circular bends, are generated, which are still laminar, steady, and showing no statistically distributed fluctuations typical for turbulent flow. This deviation from straight laminar conditions is called transitional flow, often occurring in channels larger than 500 m at higher flow rates. Transitional flow phenomena include the first occurrence of flow bifurcation, pulsating vortices, period doubling of vortex pairs, and regularly fluctuating wake flow or vortex shedding. Chaotic flow phenomena are the first evidence of turbulence. Transitional flow augments the transport characteristics in microchannels for enhanced heat and mass transfer and for performing chemical reactions in microreactors. The profound understanding of how to generate and control vortices in microchannels guides the design of advanced microstructured devices for various applications.