A review of the structure types in binary alloy sigma , mu , an X phases of transition elements is presented. Information is included on ternary alloys in which the P and R intermediate phases appear also. lt is noted that proceeding from the A-component end, the phases occurring at fixed stoichiometric ratios have structures such as at AaB the Cr/sub 3/O-type; at A/sub 2/B the Ti/ sub 2/Ni-type (eta-carbide), the MoSi/sub 2 type (C11B), and the CuAl/sub 2/-type (C16); at the equiatomic composition the CsCl-type (B2); at AB/sub 2/ the MgCu/ sub 2/-type C15), the MgZn/sub 2/-type (C14), the MgNi-type (C36), and another group of MoSi/sub 2/-type phases; at AB/sub 3/ a group of close-packed ordered CN12 structures; and at the AB/sub 5/ composition the UNi/sub 5/-type (C15/sub beta /) and the CaCu/sub 5/-type (D2/sub delta /). The emphasis in the review is on structure relations, occurrence regularities, and the effects of atomic sizes and electronic factors. (J.R.D.)