The aim of this contribution is to set the background for that portion of the Conference (some 40 per cent of the papers) which deals with transition metal complexes. Topics to be covered by other plenary lectures are omitted. Even the remaining field is much too large and I have made no effort to cover it comprehensively. Instead, my choice of fields reflects my own current interests and allows me to deal in more detail with results recently obtained by my co-workers at Glasgow. The neglect of organo-transition metal chemistry prior to 1950 was largely due to the earlier failures to obtain simple alkyls from these metals. It is appropriate therefore to begin by stressing our increased knowledge concerning alkyls. Although even simple alkyls of the earlier transition metals: Ti, V, Cr, Mn, have been isolated, their low stability readily explains the previous failures. But following the demonstration, 10 years ago, that 7r-complexing can greatly stabilize such alkyls — as for example in the alkylmanganese pentacarbonyl, RMn(CO)5, and the alkylcyclopentadienyliron dicarbonyl series RFe(C5H5) (CO)2 — the range of stable alkyls has grown steadily. A very wide range of other ligands appear capable of stabilising alkyls in addition to the carbonyl and cyclopentadienyl ligands of the early examples just mentioned. The high level of current interest in these compounds is justified by their importance in both in vitro and in vivo catalyst systems. There is strong evidence that transition metal alkyls are key intermediates in the reactions which account for the main catalytic uses of organometallic compounds: The Ziegler—Natta olefin polymerisation which is believed to involve titanium alkyls as the chain carriers; the olefin oxidations in which palladium alkyls must be at least transient intermediates; and the hydroformylation and related carbonylation processes which clearly involve alkylcobalt carbonyls. Cobalt is again involved in the only known natural organometallic system, that of Vitamin B12 in which an adenosyl group is linked through carbon to this metal. A survey of these and other cobalt alkyls (I—XII) provides excellent illustration of the wide range of ligands capable of exerting suitable stabilising influence. In compounds (I—TV) carbonyl or phosphine groups or both are used1. The bulk of the mesityl groups in compounds of the type (III) helps stability2, but is clearly not an essential factor. The 4-pyridylmethyl complex (V) is only one of numerous analogous