Donruss released this 66-card set in 1966 — the absolute origin point of Marvel trading cards. Six heroes, eleven cards each: Captain America (#1–11), Iron Man (#12–22), Daredevil (#23–33), Spider-Man (#34–44), the Hulk (#45–55), and Thor (#56–66). Card fronts used Silver Age panel art with gag captions; card backs assembled into a group portrait puzzle of all six heroes. Packs cost five cents for five cards and a stick of gum.

Fourteen cards across the set invited collectors to 'Write Your Own' caption — leaving the speech bubble intentionally blank. It was a clever marketing hook, but it created a condition catastrophe. Cards with ink filled in by children are essentially ungradeable, and because the invitation applied to cards including #3 Cap, #36 Spider-Man, and #65 Thor, high-grade survivors of those specific numbers are extraordinarily scarce.

PSA has documented only 30 total PSA 10 copies across the entire 66-card run. The centering tolerances of 1966 printing were extremely loose, bubble gum staining and wax pack contact damage are endemic, and rubber-band storage destroyed the top and bottom cards of every stored set. Card #1 (Cap) and card #66 (Thor) are the two rarest in high grade — a single PSA 9 exists of each.

Card #34 is officially the Spider-Man rookie — first card of the Spider-Man sub-series — but the artwork depicts both Spider-Man and the Human Torch together, with the caption 'Next time I'll fly the kite!' This site classifies #34 as the True Rookie for both characters. No dedicated Human Torch sub-series exists in the set, so whether #34 counts as his rookie is a genuine debate. While the card is catalogued under the Spider-Man section, both characters are clearly depicted. The Human Torch's next appearances — the 1967 Philadelphia stickers — are classified as First Solo.

The Thor sub-series (cards #56–66) contains card #61, which features Loki prominently in the artwork as a secondary figure — not as the card's named subject. Some sellers and grading companies have labeled #61 as the 'Loki Rookie.' Neither PSA nor CGC has formally assigned it a first appearance designation. This site classifies Loki's #61 as Unconfirmed Rookie. His 1975 Cropan sticker (#45) is his First Solo card, and his 1976 Topps stickers are his First English Solo appearance.

Much of the card artwork was adapted from existing Marvel comics — Ditko's Spider-Man panels, Kirby's Captain America, Hulk, and Thor, Wally Wood and Bobby Powell's Daredevil, and Don Heck's Iron Man. However, some cards feature artwork that does not cleanly match any known published comic page, suggesting Donruss may have commissioned original or adapted art for parts of the set. The exact sourcing of every card image has never been fully documented.