The Mad fold-in is a feature found on the inside back cover of virtually every Mad magazine since it was introduced in 1964. Drawn by Al Jaffee, these fold-ins are one of the most well-known aspects of the magazine. The feature was conceived in response to fold-outs in adult magazines, namely Playboy .

A Mad fold-in consists of a single drawing, with a paragraph of (often stilted and awkward) text underneath, and a panel across the top with a question. Each fold-in also features instructions on how to manipulate the fold-in, as well as a picture illustrating the procedure. Under the instructions are two arrows labeled 'a' and 'b'. When the paper is folded so that points 'a' and 'b' are touching, the text under the picture becomes the answer to the question, and the picture itself changes into a fresh image reflecting the new text.

Though a clever gimmick, experienced readers can usually estimate what the revealed picture and text will be, even before folding the page. As a buffer to this, there is sometimes a "decoy" fold-in: two pieces of a diversionary image that look as if they will combine in the fold in, but which prove to be just part of the background, or which are obscured by the fold. However, the many parodies and homages to Jaffee's fold-in construction highlight the unique dexterity he brings to the format.

The fold-in has rarely been moved since its debut. In the annual "20 Worst of the Year" issue, the fold-in is used as one of the 20 items, and appears as an internal page of the magazine. Jaffee's fold-in has also been skipped in a few issues: it was not included in issue #121 in 1968, and also skipped #190 in 1977, although Jaffee still wrote and drew the magazine's back cover. Another exception was the Mad Disco special issue published in 1980, which did not include a Fold-In due to the fact the magazine was published with stiff covers, making its use impractical.

From the feature's introduction until issue #118 (April 1968), the Fold-In was printed in black-and-white. With issue #119 (June 1968) Mad changed its printing process to allow the inside front and back pages to be printed in color; subsequently the Fold-In was presented in color from then on.