Final Fantasy VII/Summon Materia

Summon (red) materia is used to summon several different beasts, each dealing various types and amounts of damage. Summon materia is acquired at different points throughout the game, as outlined in each creature's (or materia's) individual section.

All of the summon materia have 5 stars; each star allows the creature to be summoned in combat once. Thus one-star summon materia can be used once in combat and 5-star one (i.e., mastered one) five times. Master Summon materia can be used to summon all creatures without any limitations.

Choco/Mog
Got from the Chocobo Farm. Talk to the chocobos, and choose the top dialog option ("Wark"). Watch the chocobos dance and you'll receive it.

Shiva
Get from Junon after resuscitating Priscilla.

Ifrit
Got after fighting Jenova-BIRTH in the Shinra cargo ship.

Ramuh
Get at the Gold Saucer in the chocobo jockey room when trying to win freedom from the desert prison. Note: This must be obtained at that time; it cannot be got later.

Titan
In the abandoned Gongaga Reactor which can be accessed any time after getting out of the Desert Prison. Go to the reactor area and the Turks will show up; after the Turks leave, inspect the hole they looked into in the reactor casing.

Leviathan
Solve Yuffie's pagoda sidequest and Godo gives it to you.

Odin
In the Shinra Mansion in Nibelheim. Open the safe upstairs using the clues found all over the place, and fight the boss Lost Number.

Bahamut
In the Temple of the Ancients after defeating the Red Dragon.

Kjata
Found floating around in the Sleeping Forest.

Alexander
Found in the Great Glacier, in a cave somewhere on the east side of the big snowfield. Need to touch the hot springs elsewhere first, then come to the cave and fight the person guarding it.

Neo Bahamut
Found in the Whirlwind Maze.

Phoenix
Found on the top of Fort Condor after the final battle for the Huge Materia.

Hades
Found in the sunken Gelnika.

Typhoon
Found high in the branches in the Ancient Forest near Cosmo Canyon.

Bahamut ZERO
You need to have Bahamut and Neo Bahamut materias, as well as the Huge Blue Materia. Touch the Huge Blue Materia in Bugenhagen's laboratory to get Bahamut ZERO.

Knights of Round
Found on the Round Isle, in the northeastern corner of the world. It is not marked on the map. This island cannot be landed on, so you will need a Golden Chocobo to access it.

Master Summon
The wielder of Master Summon is able to use all Summon spells, and it takes only one Materia slot. It can be obtained by getting each of the other Summon Materias to master level and touching the Huge Red Materia in Bugenhagen's laboratory. Alternatively, it can be obtained by defeating the Emerald Weapon and trading the Earth Harp on Kalm.