All units must adopt a formation. The type of formation a unit adopts will influence how it acts in battle; how it moves, how it fights, and so on. Each type of formation has its own rules.

The type(s) of formation a unit can adopt are indicated by a special rule of the same name. Models with more than one such special rule may choose their formation during deployment, and may change it and adopt a different formation by reforming during the game.

Close Order formations are by far the most usual. Therefore, the core 'how to play' rules focus upon such units. Examples of more unusual types of formation include Skirmish and Open Order, the rules for which are covered in the advanced rules section. Additionally, some army lists introduce special formation types unique to them.