Epithelial and endothelial tubes are the fundamental structural unit of organ design: most internal organs, including the vascular system, lungs, kidneys, gastro-intestinal tract and urinary-genital tract, are composed of single, or networks of, tubes that transport gases or liquids. In many cases, these tubes provide a barrier that separates different biological compartments (often the ‘outside' from ‘inside' of the organism) and regulate ionic homeostasis by vectorial transport of ions and solutes across the barrier from one compartment to the other. In addition, the sizes and shapes of tubes are precisely specified during development and modulated by physiology to generate tubular networks with the desired flow properties.