Lipid membranes under shear or tension can form surprising, cylindrical structures called membrane tubes with diameters varying between a few hundreds of nanometers to a few tens of nanometers. These structures can be formed in multiple ways, and provide a clear signature of membrane fluidity and elasticity. In vivo, tubular structures are used during intracellular transport to exchange material between compartments, and their formation depends upon the same principles as in vitro. Recent studies of the specific physico-chemical properties of membrane tubes have shed light on how tubular structures are formed in vivo. In addition, the controlled formation of such membrane tubes in vitro has proven to be an elegant way to study many dynamic processes during membrane trafficking.