Models for the melt → amorphous solid transition and for the structure of amorphous solids in metallic systems will be reviewed. Experience suggests that the tendency toward formation of metallic glasses is greatest in systems composed mainly of noble or certain transition metals (A) admixed with smaller amounts of certain metalloidal or electropositive elements (B). Often the formation of the liquid states of these AB alloys is accompanied by heat evolution, excess entropy of mixing or some volume contraction. The effects of composition on alloy glass forming tendency will be discussed in view of these data and in terms of the structural models alluded to. We note also that the occurrence of ultra-fast impurity diffusion is in alloys where a B type element is the crystalline host and an A type element is the impurity. Experience shows quite conclusively that this rapid transport is effected primarily by interstitial type defects, which must, contrary to the empirical atom size ratio rules for solution behavior, be constituted by substantial proportions of the A component. The recent internal friction studies of Turner and associates suggest that in the lead-noble metal systems the defects have off-center interstitial configurations. We suggest that this interstitial solution behavior may stem from the same type of A-B interactions which are responsible for the strong glass forming tendency in alloys where A is the host. These interactions will be discussed in view of current correlations of alloying behavior