According to Bratman's influential theory, intentions have to be viewed as more or less detailed plans. Such plans are typically high-level as far as the remote future is concerned: they are not worked out in detail and are progressively refined in order to provide basic actions that are directly executable. Inspired by Shoham's database perspective, we view basic and high-level intentions as organized in an agenda that specifies the temporal intervals within which the corresponding actions have to be performed. The agenda moreover also contains beliefs about the environment in terms of beliefs about external events. High- and lower-level intentions are linked by the instrumentality relation, alias means-end relation. We show the fundamental role of instrumentality both in the refinement of intentions (passing from high-level to lower-level intentions) and in the revision of intentions due to new information about the environment.