If your ancestors ever traveled abroad, they may have applied for a passport. Before the 1940’s, passports were not required for international travel except during times of war (Civil War and WW I, particularly). However, many Americans applied for passports even when they were not required to do so. More than a half million passports were issued between 1810 and 1909.
Passport applications generally included the birth date and birth place of the applicant (with a sworn affidavit from someone who knew them personally), plus a physical description (height, weight, hair color, etc.). Later passports included photos. An entire family might be listed on a single passport. A woman might be listed on her husband’s passport. Missionaries who served abroad often applied for passports.
Fold3 and Ancestry.com both have indexed collections of US passports. FamilySearch has unindexed passport collections from Portugal and Spain. To see a list of passport collections, type "passport" in the [Filter by collection name] box in the upper left corner.
To learn more about using Passport Applications in your family history research, stop by the Family History Center.