  This is a review of the theory of the modulational instability in idealised
fluid models of strongly magnetised plasmas and reduced models of geophysical
fluid dynamics, particularly the role it plays in the formation of zonal flows.
The discussion focusses on the Charney-Hasegawa-Mima and Hasegawa-Wakatani
models. Particular attention is paid to the wave turbulence - zonal flow
feedback loop whereby large scale zonal flows which are initially generated by
modulational instability of small-scale drift/Rossby waves tend to subsequently
suppress these small scale waves by their shearing action. This negative
feedback can result in a dynamic equilibrium in which large scale zonal flows
grow by drawing energy from small scale turbulence but suppress the small scale
turbulence in the process until a balance is reached. In this regime, the level
of small scale turbulence is greatly reduced compared to the level one would
observe in the absence of the zonal flows.
