Keywords: wormlike micelles, hydrodynamic instability, micellar aggregation reactions Abstract: The viscoelasticity of a wormlike micellar fluid derives from the presence at the microscale of long tubelike surfactant aggregations (micelles), which occur at low concentrations due to the presence of an organic salt/cosurfactant. Without this salt the micelles are spherical, and the fluid is completely Newtonian. I will present recent experiments on hydrodynamic instabilities caused or modified by the formation of a viscoelastic micellar layer between two Newtonian liquids containing either the surfactant or the organic salt. The micellar "reaction" produces a fragile material which grows in competition with local stretching or advection at the interface, leading to new effects in Hele-Shaw fingering, in droplet sedimentation, and in microfluidic emulsions.