The 1900s were a decade of transformation for the world, but especially for the United States, which saw the rise of motion pictures, motor vehicles, and experimental flight; the influence of presidents Roosevelt and Taft; and disasters such as the San Francisco earthquake of 1906. This new title, in two volumes, includes essays on these important events and more, such as industrial advances, the acquisition of the Panama Canal, sporting events such as the first World Series, and Einstein’s theory of relativity. Essays also cover topics of political ideology such as Roosevelt’s “big stick” method of foreign diplomacy, and various treaties of the time, as well as social justice issues affecting the lives of everyday Americans.

Every essay begins with a concise title followed by a brief description of the person, organization, work, concept, or event. Further headings attribute authorship, establish a date for the subject, and assign a location. Every essay features an "Impact" section, which examines the subject's broader importance during the 1900s. Special features include hundreds of images, maps and sidebars, timelines, tables and graphs, and facts and trends. To help researchers search through the content, the end of the title includes a List of Entries by Category and a detailed Subject Index.