Fusion Summon

A Fusion Summon (融合召喚 Yuugou Shoukan) is the act of Special Summoning a Fusion Monster from your Extra Deck using "Polymerization" or any other card effects which explicitly states that the Summon is a Fusion Summon.

How to Fusion Summon
To Fusion Summon using "Polymerization" you must have the Fusion Material Monsters listed on the Fusion Monster in your hand or on your side of the field, or a combination of both. You must then activate "Polymerization", select the Fusion Monster from your Extra Deck, send the Fusion Materials to the Graveyard and Fusion Summon the monster to your side of the field.

For example, to Fusion Summon "Flame Swordsman" using "Polymerization" you must have "Flame Manipulator" and "Masaki the Legendary Swordsman" (the Fusion Materials) on either on your side of the field and/or in your hand. You then activate "Polymerization", select "Flame Swordsman" from your Extra Deck, send "Masaki the Legendary Swordsman" and "Flame Manipulator" to the Graveyard and Fusion Summon "Flame Swordsman".

Advanced rulings
If a monster is Special Summoned from your Extra Deck by a card effect it is not treated as a Fusion Summon, unless stated on the card. For example Special Summoning a monster from your Extra Deck using "Cyber-Stein" is not considered a Fusion Summon.

Normally, Fusion Monsters cannot be Special Summoned from the Graveyard or Banished Zone unless they were properly Fusion Summoned before being sent. You cannot use "Re-Fusion" to revive a Fusion Monster that has been Special Summoned using "Cyber-Stein" or sent to the Graveyard by "Fusion Guard". Fusion Summoned Monsters can be revived using cards such as "Re-Fusion" unless stated otherwise.

Alternate methods
Other than using "Polymerization", cards can be Fusion Summoned using any of the following cards. Be sure to follow the instructions on the card to Fusion Summon the monster correctly.

TCG/OCG:

Anime Only:

Fusion Substitute Monsters

 * See main article Fusion Substitute Monsters.

In the anime
In the second season of the Yu-Gi-Oh! second series anime, it is stated that Fusion Monsters cannot attack the same turn they are Summoned. This is exemplified in episode 65, when Strings uses "Quick Attack" on "Humanoid Worm Drake" to bypass that rule in his Duel against Yami Yugi. This rule does not exist in the OCG/TCG and was phased out the anime for Yu-Gi-Oh! GX and Yu-Gi-Oh! 5D's.