We've talked before about the extra work that goes along with writing fantasy; the world-building and such that is in addition to the regular plotwork and so forth that any writer has to do to write in any genre.  One of those things that a fantasy writer must deal with is the rules of magic.

Almost all fantasy novels include magic of some type.  Whether it's wizards wielding fireballs or commoners wielding enchanted weapons, fantasy worlds are rife with magic.  It's part of the appeal of fantasy.  For many fantasy writers, it's also part of their downfall, at first.

Because the fantasy writer gets to make all the rules, it's easy to succumb to the temptation to start out with no rules.  Especially with magic.  Why should I have to make rules about magic?  It's MAGIC, it's supposed to be able to do ANYTHING.  That's why it's MAGICAL.

I disagree.  The magic in your fantasy world should always have limits and costs.  Often, for every magic you create you will want to create a counter, whether it is well-known to your characters or not.  But at the very least, limits and cost.

Why?  Well, think about it this way.  If your magic is all-powerful, capable of anything, and costs nothing to use, the first person to invoke it wins.  End of story.

"And Galdad the Great snapped his fingers and Hured the Evil, menace to all the free peoples of the worlds, was reduced to a small pile of smoldering ash.  Everyone cheered and they all lived happily every after."

Ick.  If there are no limits and no cost, there is also no story.  Story is about conflict and struggle, and if your magic can do everything for you and there's no reason not to use it, then there is not conflict, and no struggle.  No story.

So.  Limits.  Limits define your magic.  What can it do?  Perhaps more importantly, what can it NOT do?  If you want your characters to be able to summon spirits but not raise the dead, limit them.  If characters should be able to see the future but not scry what is happening right now somewhere else, you need a limit.  Maybe your characters really do have all-powerful magic, but they have to be touching their target.  Ach.  Limits allow your struggle to exist, by making sure your characters can't just cast a spell and make everything wonderful again.

Cost.  A well-defined system of magic should have a price that is extracted for using that magic.  Unless you really do want your characters to be able to use magic all the time.  Sometimes, if your limits are strictly defined you can get away without a cost.  If a magician must use a wand or a staff to cast magic, then cost becomes perhaps less of an issue--if you don't want him using magic you can separate him from his implements.

Usually, though, you will have a cost associated with magic.  Maybe it saps the user's strength, maybe it takes hours, days, or even years off of a person's life, depending on the magnitude of the spell.  Maybe sacrifices must be made.  Perhaps difficult to locate, expensive ingredients must be combined into a potion that is used when the magic is cast.  Cost allows your basic conflict to exist, by ensuring your characters don't use magic to solve every problem they encounter.

There are many, many other things you must consider when designing complex, believable magic systems, but the two primary things are limits and cost.  Start with those and you will be well-equipped to pursue the others.