INFORMATION concerning phase equilibria, such as can be represented by equilibrium diagrams is important in a number of fields of scientific study, and especially in metallurgy, ceramics, and chemistry. Materials of interest in these fields range through single-component to multi-component sys­ tems, the latter often being very complex. While many industrially im­ portant systems can be represented adequately by binary equilibrium diagrams, ternary diagrams provide a basis for studying a wide range of the more complex systems, such as are encountered in certain industrial alloys, and in slags and ceramics. A number of texts dealing with ternary systems are already available (see for example references 2-8). Some of these are very comprehensive and include many diagrams, and their use will take the student of the sub­ ject to an advanced and detailed level of understanding. The present mono­ graph is intended primarily as an introductory text, which it is hoped will prove useful for undergraduate and postgraduate students of metallurgy and ceramics, in particular. Special attention is given to the requirement of the metallurgist and ceramist to use phase diagrams as a means of under­ standing phase changes that occur during heating and cooling, as in solidi­ fication and heat treatment of alloys. This aspect is emphasized in terms of the principles of solidification reactions in relation to solid state constitu­ tion, particularly making use of liquidus and solidus projections.