Equip Spell Cards are placed in the Spell & Trap Card Zone of the activating player; other Equip Cards are placed in the Spell & Trap Card Zone of the player whose effect caused the card to be treated as an Equip Card. Equip Cards can be equipped to monsters on either player's side of the field. If a Monster Card is treated as an Equip Card, it is no longer treated as a monster; if a Spell or Trap Card is treated as an Equip Card, it is still treated as a Spell or Trap Card, respectively.
A monster can be equipped with any number of Equip Cards, as long as there are Spell & Trap Card Zones for these Equip Cards.
Only face-up monsters can be equipped with Equip Cards. An Equip Card cannot remain face-up on the field if it has no face-up target that is treated as a monster; if at any time a monster equipped with any Equip Cards is no longer a face-up monster (such as by being flipped face-down, leaving the field, or being treated as a Spell/Trap Card), all cards equipped to it are immediately destroyed.
Cards that equip themselves target a monster upon activation, and are then are equipped to the monster they targeted, upon resolution. If the target is no longer valid when an Equip Card resolves, it is destroyed (or remains in the Graveyard or Banished Zone if activated from there).