The development of open microfluidics is relatively recent and is an emerging sub-domain of capillarity, with many applications. This second edition research text presents the state-of-the-art theory of open microfluidics, including inertial and viscous regimes, uniform channels and converging/diverging channels, networks, bypasses and valves. It recalls the conditions for the establishment of an open microflow, and presents the dynamics of open microflows, guided by different solid structures such as fibres and threads, with a focus on open-channels. The book shows how the Lucas-Washburn law must be adapted to describe the dynamics of open microflows. It also demonstrates how surface energies, fluid properties, and solid geometry are combined to design open-microfluidic systems and devices that are used in numerous domains. Additionally, the book shows how biomimetics has inspired new advanced open microfluidic designs.