  The micro-bunching instability is a longitudinal instability that leads to
dynamical deformations of the charge distribution in the longitudinal phase
space. It affects the longitudinal charge distribution, and thus the emitted
coherent synchrotron radiation spectra, as well as the energy distribution of
the electron bunch. Not only the threshold in the bunch current above which the
instability occurs, but also the dynamics above the instability threshold
strongly depends on machine parameters, e.g. accelerating voltage, momentum
compaction factor, and beam energy. All this makes the understanding and
potential mitigation or control of the micro-bunching instability an important
topic for the next generation of light sources and circular $e^+e^-$ colliders.
This contribution will give an overview of the micro-bunching instability and
discuss how technological advances in the turn-by-turn and bunch-by-bunch
diagnostics are leading to a deeper understanding of this intriguing
phenomenon.
