'''?''' usually signals an unknown {{Distinguish|"[[ATK]], [[DEF]] or [[Level???]] value."}}{{Show card image|Orichalcos Shunoros}}

If '''?''' is a value used for a [[Monster CardsCard|monster]]'s]] [[Original ATK]] and DEF|original ATK and/or [[DEF]] to indicate that the value is represented as a '?' undefined while not on the card, it cannot be searched from the [[Main Deck|Deckfield]] using the relevant . Less frequently, a value of ? can be used in some video games for other stats with an unknown value (e.g. , such as a [[SanganMonster Token]] and [[Magician's Circle]]), since it does not have a value whilst in the [[Main Deck|DeckType]].

LikewiseWhile on the field, cards an original [[ATK]] or [[DEF]] value of ? is always treated as 0. Almost all monsters that search the use such a value will have an [[Graveyardeffect]] using a that increases the relevant stat or sets it equal to some value cannot . The only three exceptions are "[[Holactie the Creator of Light]]" (with makes its controller win the Duel when Summoned, making its ATK/DEF nearly irrelevant), "[[The Winged Dragon of Ra - Sphere Mode]]", and "[[Number C1000: Numerounius]]" (whose stats are intended to be used (e.g. set by "[[Limit ReverseNumeron Chaos Ritual]]").

If a While in the [[Monster Cards|monsterhand]] has ? , [[ATKMain Deck|Deck]] , or [[DEFGraveyard]] and the , a monster with an ATK or DEF of ? cannot be searched or otherwise used by card effects that require a defined ATK or DEF, such as "[[Card effect|effectsSangan]] that determines the ", "[[ATKMagician's Circle]] or ", "[[DEFLimit Reverse]] cannot be ", and "[[activateSkull Conductor]]d or ". In addition, a monster whose ATK and DEF are both ? is not considered to have [[negateequal ATK and DEF]]d while not on the [[Playing Field|field]], its ATK or DEF is treated as 0.

Also on '''????''' and '''X000''' are equivalents used uniquely for the field[[illegal]] versions of "[[The Winged Dragon of Ra (original)|The Winged Dragon of Ra]]" and "[[Slifer the Sky Dragon (original)|Slifer the Sky Dragon]]". In the ''TCG'', if an effect asks for Original ATK, then the Original ATK illegal version of a monster with "The Winged Dragon of Ra" in ''[[Yugi's Legendary Decks]]'' uses '''?' ATK is treated as 0 Original ATK'' instead.

In the TCG, if a card that affects the ? ATK and DEF are not used in ''[[HandYu-Gi-Oh! Rush Duel]]'', and counterparts of ''TCG''/''OCG'' monsters with those values, such as "[[Deck Devastation VirusMaju Garzett (Rush Duel)|Maju Garzett]] is activated", the monsters replace it with ? [[ATK]] are treated as 0. However in the OCG, the monsters with ? [[ATK]] are still treated as ?. So [[Deck Devastation Virus]] can destroy them in hand in the TCG, but not the OCG.