A catalyst for addition polymerization comprising(a) an (initiator) first component comprising a plurality of atoms M as hereinafter defined, each in a moiety of formula (I):MX(Y.sub.n)Z.sub.p (I) whereinM is Si, Ge or Snn is 0 or an integer, and p is an integer such that (n+p)=3;X is a diradical ##STR1## bound via Q to M whereQ is N, P, As or P(=T)G.D where T is O or S and G and D are each independently a bond, O or S,R.sup.1 is optionally substituted hydrocarbyl, or, when Q is P, M(Y.sub.n)Z.sub.p as herein defined,R.sup.2 is optionally substituted hydrocarbadiyl, orR.sup.1 and R.sup.2 together are hydrocarbatriyl,all such R.sup.1 and R.sup.2 being inert in the conditions of the process of the present invention; andR.sup.2 being linked directly or indirectly to any group as defined for X, or an organic radical comprising further moieties of formula (I), or one or more M(Y.sub.n)Zp moieties as herein defined; andY and Z are independently any group A or OA where A is optionally substituted hydrocarbyl inert in the conditions of the process of the present invention, or Y is optionally substituted trialkylsilylalkyl; and(b) a (co-catalyst) second component salt comprising an anion selected from azide, cyanide, cuanate, fluoride, bifluoride, nitrate and optionally substituted organic mono- and poly-phosphonates, -phosphinates -sulphonates, -sulphinates, -carboxylates, -siloxides and -oxides; anda cation which in use of the catalyst is inert under the polymerization process conditions yet renders the co-catalyst available in the polymerizing medium; initiator components for such catalysts, compositions comprising such components, and a polymerization process, in particular the addition polymierization of vinylic monomers, using the catalysts.