Due to rapid improvements in on-board instrumentation and atmospheric observation systems, in most cases, aircraft are able to steer clear of regions of adverse weather. However, they still encounter unexpected bumpy flight conditions in regions away from storms and clouds. This is the phenomenon of clear air turbulence (CAT), which has been a challenge to our understanding as well as efforts at prediction. While most of such cases result in mild discomfort, a few cases can be violent leading to serious injuries to passengers and damage to the aircraft. The underlying physical mechanisms have been sought to be explained in terms of fluid dynamic instabilities and waves in the atmosphere. The main mechanisms which have been proposed are:

1. Kelvin–Helmholtz instability of shear layers,

2. waves generated from flow over mountains,

3. inertia-gravity waves from clouds and other sources,

4. spontaneous imbalance theory and

5. horizontal vortex tubes.

This has also undergone a change over the years. We present an overview of the mechanisms proposed and their implications for prediction.