This publication contains the proceedings of the second technical meeting on the theory of plasma instabilities, initiated by the IAEA. It presents the latest work of the 49 participants. Many different types of plasma instabilities have been found in plasma devices. These have been partially explained by theoretical models, but knowledge is still incomplete and has to be supplemented by further theoretical and experimental efforts. Plasma turbulence results in cross-field transport of one or two orders of magnitude larger than the transport arising from neoclassical transport due to binary collisions. The consequent reduction in plasma performance has implications for the development of fusion energy as an economic alternative power source. Thus a comprehensive understanding of plasma turbulence remains an important scientific objective.