In this paper theoretical and experimental studies of the phase behaviour of ternary polymer blends are reviewed. Particular emphasis is placed upon the effect of specific interactions (hydrogen bonds). The association model developed to predict the phase behaviour of binary hydrogen-bonded polymer blends has been extended to ternary polymer blends. Simulations have been performed to illustrate the major effects of ’physical’ (primarily dispersion) and ’chemical’ (hydrogen bonding) forces on phase behaviour. Theoretically predicted phase diagrams have also been compared to experimental results obtained from five hydrogen-bonded ternary polymer blend systems. Overall, the agreement is satisfactory. One general conclusion is that it will be very difficult to find ternary polymer blends that exist in a single phase over a wide composition range. Furthermore, in most cases an immiscible binary blend cannot be made homogeneous by introducing a small amount of a third polymer (’compatibilizer’).