The Additions, accessioned between 1984 and 2001, help fill in several gaps in documentation present in the original collection, particularly in regards to William Steiger's early political career in the Wisconsin State Legislature and his 1966 Congressional Campaign. While not extensive, the additions also shed light on Steiger's late teenage years, his early involvement in Republican Youth Politics, and early travel experiences.

The arrangement generally that of the original collection; where applicable, chronological and alphabetical arrangements were also employed.

These additions have been weeded at the folder level to remove large collections of clippings, as well as general publications and pamphlets without any personal notations by Steiger or his office.

The Reference Files series, 1955-1975, contains background information related to legislation and issues with which Steiger was involved. The bulk of the materials are from William Steiger's time in the Wisconsin State Legislature. Files may contain correspondence, reports, memos, speeches, surveys, statistics, bills, and expense reports on a variety of subjects including reapportionment, redistricting, election laws, voting, and human rights.

This series is arranged chronologically by congress and then alphabetically by subject.

The Legislative Constituent Correspondence series, 1961-1966, contains constituent correspondence from Steiger's tenure in the Wisconsin State Legislature. The series is arranged chronologically and then alphabetically by constituent.

The Publicity and Information series, 1966-1978, contains press releases, speeches, statements, memos, correspondence, and promotional materials created during Steiger's time in Congress. Speeches and statements cover a variety of topics including workforce training, congressional reform, the volunteer army, and education.

This series has been arranged chronologically by congress and then alphabetically topic.

Of note, is one carton entirely related to Steiger's 1966 Congressional campaign which contains correspondence, news and press releases, and other campaign records. The arrangement of this carton remains, as it was received, by topic.

The Committees series, 1967-1976, contains correspondence, memos, reports, mailings, questionnaires, minutes, meeting agendas, and rules created during Steiger's time on the Education and Labor Committee (1967-1971) and the GOP “Rule 29 Committee” (1972-1976), which determined who could be a delegate to the Republican National Convention, and for which Steiger was the chairman. The series is arranged by committee.

The Personal Files series, 1941-1979, contain materials created and collected by Steiger primarily from his late teens through his early years in Congress. Materials include correspondence, fliers, notes, high school essays and writing projects, letters, and personal financial records. Includes materials from several organizations with which Steiger was involved prior to being elected to Congress in 1966, including Badger State Boys, Young Republicans, the Lutheran Home of Oshkosh, the Madison Club, and the YMCA. Also contains an extensive letter detailing Steiger's trip to Europe in 1958 which included trips to England, Finland, France, Germany, and the Soviet Union; and two folders of condolence letters written to Steiger's family after his death in 1978.

This series has been arranged chronologically and the alphabetically by subject.

The Photograph series are from Steiger's campaigns (primarily from 1966); his initial swearing-in; and his work as a congressman in Washington, D.C. and in his district. Also included are photos of his political activities as a youth (including his election at Governor of Badger Boys State) as well as photographs of other politicians Steiger admired. A volume of contact sheets corresponds to negatives held at the Wisconsin Historical Society in Madison.

Also included are three volumes of family photos including images of Steiger's mother, Ruth Steiger (née Storms), at several northern Wisconsin locations including Campfire Girls camp in Ephraim and images from her years as a student at UW Madison including her experiences in the Delta Gamma sorority. The third volume contains images of Steiger's father, Carl Steiger including images from his tenure on the UW System's Board of Regents.