Summary: WITH its first issue, 1925-27, the “World List” took rank as a notable addition to library resource. It essayed to catalogue the scientific periodicals of the world current at commencement of this century and thence onward to 1921. To each of the periodicals (more than 25,000) of its list it assigned a reference-title individually distinctive. Further, for each of those traceable to any of 150 given representative libraries in the British Isles, the list stated by which of those libraries the periodical was filed. This census of extant periodicals, and of the British intake of them, was undertaken at a significant time. Want of it was being acutely felt. A post-War world in being was making departures fresh. In various directions the recovery of scientific production was almost feverish. Renascent nations newly risen rejoiced to sho\y their virility and culture by contributing to science through channels natively and linguistically their own. However desirable might be a full inventory of all this activity, it was far from easily compassed. It meant search at sources emanating the world over. Their number proved to be yet greater than had been thought. Nevertheless the difficulties were overcome, and the results justified their undertaking.