Focus is regularly treated as a cross-linguistically stable category that is merely manifested by different structural means in different languages, such that a common focus feature may be realised through, for example, a morpheme in one language and syntactic movement in another. We demonstrate this conception of focus to be unsustainable on both theoretical and empirical grounds, invoking fundamental argumentation regarding the notions of focus and linguistic category, alongside data from a wide range of languages. Attempts to salvage a cross-linguistic notion of focus through parameterisation, the introduction of additional information-structural primitives such as contrast, or reduction to a single common factor are shown to be equally problematic. We identify the causes of repeated misconceptions about the nature of focus in a number of interrelated theoretical and methodological tendencies in linguistic analysis. We propose to see focus as a heuristic tool and to employ it as a means of identifying structural patterns that languages use to generate a certain number of related pragmatic effects, potentially through quite diverse mechanisms.