A player may activate and Set as many Spell Cards as they wish during their turn. All Spell Cards are Spell Speed 1 (meaning they can only be activated as Chain Link 1), with the exception of Quick-Play Spell Cards which are Spell Speed 2 (meaning they can be activated in response to Spell Speed 1 and 2 Chain Links). Spell Cards, except Quick-Play Spell Cards, can only be activated during their possessor's Main Phase (except "Curse of Fiend"); Quick-Play Spell Cards can be activated from their possessor's hand during their turn, or while Set on the field during either player's turn.
After activation, Spell Cards (with the exception of Continuous, Field and Equip Spell Cards) are sent to the Graveyard.
Some very powerful Spell Cards have been Forbidden from Advanced Format tournament play, such as "Pot of Greed". In many cases, Konami has released similar cards with less powerful or more restrictive effects to serve as replacements for some of these Forbidden cards. For instance, "Pot of Greed" has been replaced by the weaker "Pot of Desires", which provides the same bonus as "Pot of Greed" but comes with a considerable activation cost (whereas "Pot of Greed" has no activation cost at all).
Used properly, a single Spell Card can significantly alter the game in the user's favor, or even cause them to win the Duel.