Archives traditionally include collections made up of physical items: personal papers, unpublished manuscripts, photographs, correspondence, and so much more. Digital preservation of archival documents is now underway at many repositories, and accelerating every day, making remote access possible. However, the vast majority of archival materials are still not yet digitized, which is important to keep in mind as you search online.
We have a thorough and detailed research guide on archival research. See the Finding Digitized Collections page of the guide for information on finding and using digitized primary sources. There are links to online portals, library databses, selected research librariies with digitized materials, international resources, and more.