This is a combined volume of the Proceedings for the Workshop on 'Mechanisms of High Tc Superconductivity' (20 June -29 July) and the Adriatico Research Conference (26 - 29 July) on high temperature superconductivity. It aims to bring together a critical overview of the experimental situation and an in-depth discussion of the various theoretical models proposed to explain the mechanisms for high temperature superconductivity by experts actively involved in high Tc research.