Spellcasting Systems are almost certainly the most complex part of most Roleplaying games. Certainly where they are included they are the most complex part of the game that they are in. The problem is that the game must attempt to model something that has no precedent or parallel in the real world. The Game Designer is free to come up with any metaphor or mechanic for magic that he likes, and to indulge himself in the invention of a system to make it work in the game. We all have some idea of how combat or lock picking might work, and of what factors need to be taken into account, and thus combat systems are not only simpler, but are very similar in most systems. Magic, however, is almost never the same.

Nevertheless there are certain common aspects that crop up time and again, and various mechanics used in a number of games.

Spells vs. Freeform Magic

The most obvious system for magic, and the most used, involves writing a list of pre-defined Spells, magic formulae with a fixed effect and varying powers. Some systems have but a few (e.g. Machineguns and Magic's 30) while others have vast numbers (Basic Termite 2.0 has 450 spells, while the entirety of AD&D has more spells than can be counted spread over many volumes). In either case the principle is the same, a Mage can employ one or more spells a day/week/hour each of which will have an effect defined in the rules. This makes the magic system easy to understand, and it makes the spells varied and interesting. It also parcels magic up into chunks which can be quested for and hidden. Indeed some systems (Such as Call of Cthulhu) only allow spells to be acquired if they can first be found, taught, or stolen.

The alternative (as used in Termite Edition 1.0, Atlantis, Amber, Ars Magica and Mage) is to make magic freeform, i.e. to allow the Mage to create his spell effect 'on the fly' using only guidelines presented in the rules. This is a much less popular choice just because it takes much longer to run and is much more difficult to construct. The game designers must try and work out in advance the things Mages may want to do and provide rules for them, and the GM must constantly adjudicate whether or not a character can do something, and how much it will cost to do. It my well be that such systems better suit particular styles of Fantasy Novel (Such as Amber or The Belgariad) but they are certainly much more challenging to game.

Limited Magic vs. Unlimited

The next choice in a spell system is whether or not the Mage works to some fixed number of spells a day, limited either by the number of spells or the spending of some sort of Magic Points (See below), or whether they can cast as many spells as they like a day until they fail a dice roll. The first system is by far the most common (Amber, AD&D, Termite, Nephilim, Changeling... etc.), but the second has notable occurrences such as Dragon Warrior's Mystics, or Machineguns and Magic.

A modification to the Unlimited method is to allow a Mage to cast an unlimited number of spells a day provided that they can summon the energy to do so, either gathering it from an external source or making some sort of roll to avoid using up the store each time a spell is cast. (Castle Falkenstein, Mage, Termite Sun-Keld).

Numbers of Spells vs. Spell Costs

In AD&D, the great-grandfather of all RPGs, a Mage can cast a fixed number of spells a day, each of a certain level, e.g. 4 1st Level spells, and 2 2nd level spells a day. Levels can't be swapped and it doesn't matter which spell is cast, only what level it is. Further once a spell is cast the mage forgets it and cannot cast it again until he has re-memorised it (though he can memorise a spell more than once). A few other games followed AD&D in giving players a number of 'spell units' to play with, but in general most games quickly shifted to enumerating magic in terms of some sort of Spell Costs or Points.

Depending on whether a system was freeform or not Points costs are either applied to each spell (Termite 2.0, Nephilim, Dragon Warriors, Warhammer, Call of Cthulhu... too many to list), or to various components or effects that go up to make the freeform magic (Castle Falkenstein, Atlantis, Mage, Ars Magica).

Magic Points vs. Hit Points & fatigue

Finally one must ask where these Spell Points, if they are being used, come from. There are two major schools of thought on this question. The more High fantasy type Games (Termite 2.0, Warhammer, Dragon Warriors) give a Mage an extra Magic Point (MP) total which is spent to cast spells and returns at some convenient time, e.g. Midnight, or after the Mage has rested. The harsher, or more realistic (realistic magic?) style of games (The Fantasy Trip, Ars Magica, Termite 2.0 as well, GURPS) use the character's Hit Points as his source of magic, i.e. casting too many spells can kill you, or at least incapacitate you. Since this is very harsh most systems apply the damage in the form of Stun or fatigue, which will not usually kill the character, but will instead exhaust, weaken, and even knock out the Mage.

A modification of the Magic Points system has become popular with World of Darkness games. Here the Magic Points (Glamour in changeling, Mana in Mage) need not always be used for magic, only to ensure that they work in tricky situations, and they are not related to health. However these points do not come back quickly, as most system's magic points do, but may take days of game time, or many sessions to actively restore. These systems might be termed Active Mana systems, since the player must choose to spend the points and must work to regain them, while most systems are Passive Mana systems, where points are always used to cast spells and where the points return automatically after a set period.