Microforms are any form, either films or paper, containing microreproductions^[1] of documents for transmission, storage, reading, and printing. Microform images are commonly reduced about 25 times from the original document size. For special purposes, greater optical reductions may be used.

All microform images may be provided as positives or negatives, more often the latter.

Three formats are common: microfilm (reels), aperture cards and microfiche (flat sheets). Microcards, a format no longer produced, were similar to microfiche, but printed on cardboard rather than photographic film.