As part of a long-range project compiling data and source materials for American business history, we recently completed an extensive examination of vertical integration in a cross section of American manufacturing firms in the period 1899-1 948. The purpose of this examination was to determine when, where, and why vertical integration occurred in the period after the giant mergers and before the conglomerates. The purpose of this note is to present a summary of the conclusions drawn from the mass of data assembled in the study.