The invention relates to apparatus and a method for the prevention of movement of a component which is provided in engagement, typically screw threaded engagement, with an article, one such article being a body of a valve assembly. The apparatus is provided in the form of a two part retaining device. One of the parts is engaged to the articles and the other part is located with respect to the component. The parts are provided with mutual engagement means, which, when engaged, are non rotatable and which, in turn, prevent relative rotation of the component with respect to the article.