Concentration data - which expresses the relative importance of the largest firms in an industry - covering almost the entire range of British industry, were made available for this study, first published in 1960, by the Board of Trade. The authors combined with each industry's concentration-ratio with the average and relative sizes of its constituent firms and plants, and so sought to determine its structural type. Then, by comparing these results with those of an earlier study, they established in which trades significant changes in concentration occurred since 1935. Two chapters describe how the leading firms in such highly concentrated trades as sugar refining, wallpaper, matches, explosives, tinplate and oil-refining grew over the years and how they maintained their position. There is also a discussion of the relevance of such factors as mergers, nationalisation, technological changes, illustrated by reference to brief case-studies of twenty trades.