This Tutorial Review shows the behaviour of water in micro- and nanofluidic systems. The chemical potential of water ('water potential') conveniently describes the energy level of the water at different locations in and around the system, both in the liquid and gaseous state. Since water moves from high to low potential, the water potential enables us to predict and describe the direction of water movement inside systems and between systems and their surroundings. Practical examples of microfluidic devices illustrate the different contributions to the water potential (capillary, gravitational, entropic (osmotic) in liquid water; the partial vapour pressure in atmospheric water and van der Waals forces in water films) and the resulting water movement.