To join the mages guild you must first visit any one of the guilds. Then you must talk to the guilds leader, to be allowed access to the guild you must have no bounty on your head.

Perks

When you join the mages guild many things become available to you. Every mages guild has some sort of soul gems, some even have grand soul gems. So if you need a soul gem just visit one of the many mages guilds in tamerial. Also when you join the mages guild many quests become available to you. Another perk for joining the mages guild is that you can be accepted into the Arcane University, where you can create your own enchantements and spells. You can also use the beds located in the mages guilds.

How to be Accepted into the Arcane University

To join the arcane university you must get a recommendation from all the guild leaders.  Just go over and talk to them about "recommendation" and they will give you a quest. When you finish the quest you will be given there recommendation.

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Facts

Each guild specializes in a certain type of magic. At the guild halls you can purchase spells from various members of the mages guild. You can also get training about certain types of magic.

Rejoin Mages Guild

If you are suspended from the mages guild you will have to talk to Raminus Polus. He will send you on a quest to collect alchemy ingredients to get reinstated.

Quests

For a list of quest walkthroughs please visit oblivion mages guild.

Ranks

Rank	Name
	Associate
	Apprentice
	Journeyman
	Evoker
	Conjurer
	Magician
	Warlock
	Wizard
	Master-Wizard
	Arch-Mage