Electrodeposited Zn-Ni alloy coatings are of particular interest for improving the corrosion resistance of steel in a number of enviornments. Of particular interest is the relationship between composition, structure and corrosion rate. This paper firstly reviews the literature regarding composition-structure relationships of Zn-Ni electrodeposits and compares them with the equilibrium phase diagram. Secondly, research was carried out on a wide range of coatings which were produced in the laboratory and their structure and corrosion rates determined. It was found that unambiguous identification of phases from XRD data can be difficult. Maximum corrosion resistance of deposits is obtained at 12-13% Ni, with a $gamma$ phase structure and predomination of (600) and (411) reflections. Compatibility is important with regard to chromate conversion coatings.