What is linguistic diversity? How can it be measured? Can the spreading of a lingua franca be expected to reduce it? And if so, does it matter? In an attempt to answer these questions, this paper first distinguishes the three dimensions of diversity - richness, evenness and distance - and describes the tension that is bound to arise between linguistic diversity in various senses and the promotion of multilingualism. It next distinguishes between the two levels of linguistic diversity - local and territorial - and describes the structural tension that tends to develop between local and territorial linguistic diversity. Against this background, it argues (a) that linguistic diversity, by itself, cannot plausibly be regarded as a good, all things considered, especially because of its negative impact on the prospects for economic solidarity; (b) that the erosion of local linguistic diversity should therefore be witnessed with equanimity; but (c) that territorial linguistic diversity will nonetheless need preserving as the by-product of a concern for the equal dignity of the identities closely associated with native languages.