The effects of different elements on the A4 and A3 transformation points of iron are described, and a discussion given of the way in which this leads to different types of equilibrium diagram. The importance of the relative sizes of the constituent atoms in determining the structure of the alloys is emphasized, and it is shown how some of the structural characteristics are related to the relative positions of solvent and solute in the Periodic Table, although quantitative theory is lacking. The useful properties of steels generally involve structures which are not those of the equilibrium state, and some of the transformations concerned are briefly outlined.