Multiclassing is the process of of taking levels is class outside of the one you started with, trading away the consistency of a single class in exchange for combining the powers of two classes to create something interesting out of them. This page is to explain what prerequisites are necessary for each class, what you gain from them, and other factors.
To qualify for a new class, you must meet the ability score prerequisites for both your current class and your new one, as shown in the Multiclassing Prerequisites table.
Class - Ability Score Minimum
Death Knight - Strength 13, Charisma 13
Demon Hunter - Dexterity 13, Intelligence 13
Druid - Wisdom 13
Hunter - Dexterity 13, Wisdom 13
Mage - Intelligence 13
Paladin - Strength 13, Charisma 13
Priest - Charisma 13
Rogue - Dexterity 13
Shaman - Wisdom 13
Warlock - Intelligence 13
Warrior - Strength 13 OR Dexterity 13
You gain the hit points from your new class as described for levels after 1st. You gain the 1st-level hit points for a class only when you are a 1st-level character.
You add together the Hit Dice granted by all your classes to form your pool of Hit Dice. if the Hit Dice are the same die type, you can simply pool them together. For example, both the Warrior and the Paladin have a d10, so if you are a Paladin 5/Warrior 5, you have ten d10 Hit Dice. If your classes give you Hit Dice of different types, keep track of them separately. If you are a Paladin 5/Priest 5, for example, you have five d10 Hit Dice and five d6 Hit Dice.
When you multiclass into the 1st level of a class, you gain a handful or proficiencies from that class. Below showcases what each class gives you in terms of proficiencies.
Death Knight : Light Armor, Medium Armor, Simple Weapons, Martial Weapons
Demon Hunter : Simple Weapons, Martial Weapons, Warglaives
Druid : Light Armor
Hunter : Light Armor, Medium Armor, Shields, Simple Weapons, Martial Weapons, One skill from the class's skill list
Mage : N/A
Paladin : Light Armor, Medium Armor, Shields, Simple Weapons, Martial Weapons
Priest : N/A
Rogue : Light Armor, One Skill from the Class's Skill List, Thieves' Tools
Shaman : Light Armor, Medium Armor, Shields, Simple Weapons
Warlock : N/A
Warrior : Light Armor, Medium Armor, Shields, Simple Weapons, Martial Weapons
When you gain a new level in a class, you get its features for that level. A few features, however, have additional rules when you're multiclassing: Extra Attack, Unarmored Defense, and Spellcasting.
If you gain the Extra Attack class feature from more than one class, the features don't add together. You can't make more than two attacks with this feature unless it says you do (as the Warrior's version of Extra Attack does).
If you already have the Unarmored Defense feature, you can't gain it again from another class.
Your capacity for spellcasting depends partly on your combined levels in all your spellcasting classes and partly on your individual levels in those classes. Once you have the Spellcasting feature from more than one class, use the rules below. If you multiclass but have the Spellcasting feature from only one class, you follow the rules as described in that class.
Spells Known and Prepared : You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class. If you are a Shaman 4/Mage 3, for example, you know seven 1st-level Shaman spells from the Shaman spell list. As 3rd-level Mage, you know three Mage cantrips, and your spellbook contains ten Mage spells, two of which (the two you gained when you reached 3rd level as a Mage) can be 2nd-level spells. If your intelligence is 16, you can prepare six wizard spells from your spellbook.
Each spell you know and prepare is associated with one of your classes, and you use the spellcasting ability of that class when you cast the spell. Similarly, a spellcasting focus, such as a holy symbol, can be used only for the spells from the class associated with that focus.
Spell Slots : You determine your available spell slots by adding together all your levels in the Druid, Mage, Priest, and Shaman classes and half your levels (rounded down) in the Paladin class, as well as Feral Druid and Enhancement Shaman. Use this total to determine your spell slots by consulting the Multiclass Spellcaster table.
If you have more than one spellcasting class, this table might give you spell slots of a level that is higher than the spells you know or can prepare. You can use those slots, but only to cast your lower-level spells. If a lower-level spell that you cast, like burning hands, has an enhanced effect when cast using a higher-level slot, you can use the enhanced effect, even though you don't have any spells of that higher level.
For example, if you are an Enhancement Shaman 4/Mage 3. you count as a 5th-level character when determining your spell slots: you have four 1st-level slots, three 2nd-level slots, and two 3rd-level slots. However, you don't know any 3rd-level spells, nor do you know any 2nd-level Shaman spells. You can use the spell slots of those levels to cast the spells you do know - and potentially enhance their effects.
Pact Magic : If you have both the Spellcasting class feature and the Pact Magic class feature from the warlock class, you can use the spell slots you gain from the Pact Magic feature to cast spells you know or have prepared from classes with the Spellcasting class feature, and you can use the spell slots you gain from the Spellcasting class feature to cast warlock spells you know.
As you may have noticed, the classes all lack the class feature of Ability Score Improvements, which gives the class ether a +2 to one Ability Score, a +1 to two, or you gain a feat. That's because things are handled differently here.
Instead, when you reach a specific character level, which is the sum of all of your class levels, you will then gain the choice of a +2 to one Ability Score or a +1 to any two Ability Scores, or you can choose to gain one of the feats lists on the page over here. The character levels you gain this ASI at is the same as standard: 4th, 8th, 12th, 16th, and 19th level.