Proofreading is a phase in the process of publishing where galley proofs are compared against the original manuscripts or graphic artworks, to identify transcription errors in the typesetting process. In the past, proofreaders would place corrections or proofreading marks along the margins. In modern publishing, material is generally provided in electronic form, traditional typesetting is no longer used and thus (in general) this kind of transcription no longer occurs.

A "galley proof" (familiarly, "a proof") is a typeset version of copy or a manuscript document. It may contain typographical errors ("printer's errors"), as a result of human error during typesetting. Traditionally, a proofreader looks at a portion of text on the copy, compares it to the corresponding typeset portion, and then marks any errors (sometimes called "line edits") using standard proofreaders' marks.

Unlike copy editing, the defining procedure of a proofreading service is to work directly with two sets of information at the same time. Proofs are then returned to the typesetter for correction. Correction-cycle proofs will typically have one descriptive term, such as "bounce", "bump", or "revise" unique to the department or organization and used for clarity to the strict exclusion of any other.^{[citation needed]}

"Copy holding" or "copy reading" employs two readers per proof. The first reads the text aloud literally as it appears, usually at a comparatively fast but uniform rate. The second reader follows along and marks any pertinent differences between what is read and what was typeset. This method is appropriate for large quantities of boilerplate text where it is assumed that there will be comparatively few mistakes.

Experienced copy holders employ various codes and verbal shortcuts that accompany their reading. The spoken word "digits", for example, means that the numbers about to be read are not words spelled out; and "in a hole" can mean that the upcoming segment of text is within parentheses. "Bang" means an exclamation point. A "thump" or "screamer" made with a finger on the table represents the initial cap, comma, period, or similar obvious attribute being read simultaneously. Thus the line of text "(He said the address was 1234 Central Blvd., and to hurry!)" would be read aloud as "in a hole [thump] he said the address was digits 1 2 3 4 [thump] central [thump] buluhvuhd [thump] comma and to hurry bang". Mutual understanding is the only guiding principle, so codes evolve as opportunity permits. In the above example, two thumps after buluhvuhd might be acceptable to proofreaders familiar with the text.

"Double reading" is when a single proofreader checks a proof in the traditional manner and then another reader repeats the process. Both initial the proof. With both copy holding and double reading, responsibility for a given proof is necessarily shared by the two proofreaders.

"Scanning" is used to check a proof without reading it word for word, has become common with computerization of typesetting and the popularization of word processing. Many publishers have their own proprietary typesetting systems, while their customers use more common commercial programs. Before the original data can be published, it must be converted into a format used by the publisher. The end product is usually called a conversion. If a customer has already proofread the contents of a file before submitting it to a publisher, there will be no reason for another proofreader to re-read it from the copy (although this additional service may be requested and paid for). Instead, the publisher is held responsible only for formatting errors, such as typeface, page width, and alignment of columns in tables; and production errors such as text inadvertently deleted. To simplify matters further, a given conversion will usually be assigned a specific template.

Proofreaders are expected to be consistently accurate by default because they occupy the last stage of typographic production before publication.