This paper reviews the state of the art in design methods for multicomponent condensation and boiling. In multicomponent condensation, a range of methods of varying complexity are available to calculate the local heat and mass transfer rates; the complexity depends on the detail with which the mass transfer resistances are calculated. These methods are briefly reviewed and their interrelationships along with those of their expertmental validation are discussed. Mixture boiling research has thus far followed different lines and very little effort has gone into understanding the liquid multicomponent diffusion process in boiling. Research work has been mainly concentrated on understanding the dynamics of bubble growth; heat transfer calculations are done using empirical corrections to coefficients calculated for pure fluids. The extensive research in these areas is reviewed and also the few previous studies on convective two-phase heat transfer to mixtures are described. In conclusion, possible design recommendations are offered. It is suggested that, although condensation and boiling heat transfer in mixtures have hitherto been studied separately, a combined research approach may produce further advances.