Records consist of correspondence, financial records, legal documents, organizational records and other materials for the companies that made up the Montana Power Company, both before and after the organization of the Company. Records are arranged alphabetically by subgroup. The largest subgroups include:

Butte Electric and Power Company (1901-1912) records include interoffice and general correspondence, financial records, legal documents, maps, organizational materials, and reports.

Deer Lodge Electric Company (1892-1929) records include a historical information file, financial records, and organizational materials.

Great Falls Land Improvement Company (1905-1945) records include financial records and lot contracts. The Great Falls Power Company records include financial records concerning the construction of the Big Falls Development [Rainbow Dam], the Black Eagle Redevelopment, the Great Falls Development [Volta/Ryan], Holter Development, and Rainbow Extension; legal documents; and reports. Records of Great Falls Street Railway Company (1890-1936) include financial records and subject files. Records of the Great Falls Water Power and Townsite Company and its successor the Great Falls Townsite Company (1887-1936) include interoffice, incoming, outgoing, and general correspondence, employment records, financial records, lot books and other legal documents, organizational materials, and numbered subject files.

Havre Electric Company records (1904-1914) consist primarily of financial records and organizational materials.

There are records for several Helena companies which merged, separated and re-organized several times, including the Helena Electric Company, the Helena Electric Railway Company , the Helena Gas and Electric Company, the Helena Gas Light and Coke Company, the Helena Light and Railway Company, the Helena Motor Railway Company, the Helena Power and Light Company, the Helena Power Transmission Company, the Helena Steam Power and Lighting Company, the Helena Street Railway Company, the Helena Water and Electric Power Company, and others.

Records for the Lewistown Coal, Gas and Light Company and its successor the Lewistown Electric and Power Company (1907-1914) include historical information files, court papers, financial records, legal documents, and organizational materials.

The Madison River Power Company subgroup (1905-1912) includes a historical information file, incoming correspondence, financial records, legal documents, and organizational materials.

Records of the Missoula Light and Power Company and its successor the Missoula Light and Water Company (1896-1924) include incoming and outgoing correspondence, court papers, employment records, financial records, Public Service Commission hearings, legal documents, organizational materials, reports, and miscellany. The Missoula Street Railway Company records (1889-1927) include court papers, employment records, financial records, Public Service Commission hearings, legal documents, organizational materials, and miscellany, including accident reports. The Missoula Waterworks Company/Missoula Water Company subgroup (1886-1904) includes outgoing correspondence, court papers, employment records, financial records, and field books. In 1924 the various Missoula companies merged to form the Missoula Public Service Company. Records of this company (1924-1929) include court papers, employment records, financial records, and legal documents.

Records of the Missouri River Electric and Power Company (1912) include financial records, legal documents, and organizational materials. The Missouri River Power Company subgroup (1900-1910) includes general correspondence, financial records, legal documents, and organizational materials.

Records of the Montana Power Construction Company Milwaukee Road Electrification subgroup (1914-1916) include interoffice and general correspondence, employment records, and financial records of construction camps and outfit trains involved in the electrification of the Chicago, Milwaukee, and St. Paul Railroad. Records of the Phoenix Utility Company (1929-1934) include interoffice correspondence, financial records, and reports concerning the building of transmission lines and the Flathead and Morony hydro-electric projects.

The Thompson Falls Power Company subgroup (1912-1929) includes financial records, legal documents, and organizational materials.

Records of the Union Electric Company of Dillon (1908-1940) include general correspondence, financial records, legal documents, organizational materials, and reports.

Records of the United Missouri River Power Company (1906-1912) include a historical information file, court papers, financial records, legal documents, organizational materials, and reports.

There is a subgroup of Multi-company materials (1905-1927) which includes comparative financial statements for several companies, maps of transmission systems involving several companies, and statewide transmission line construction reports.

In addition there are papers for three company attorneys in Missoula: Walter M. Bickford, James M. Brown , and William L. Murphy . The Bickford papers are notable for including extensive correspondence with William A. Clark, William A. Clark, Jr., and Clark offices in Missoula and New York, many of a personal nature.