Not all units can charge. Units that are already engaged in combat, that are fleeing, or that rallied during the Strategy phase of this turn cannot declare a charge or make a charge move. Units that are in Marching Column can declare a charge, but cannot make a charge move. In rarer cases, units may be prevented from either declaring a charge, or making a charge move, by a special rule or spell effect.

Additionally, a unit cannot declare an impossible charge – i.e., one that it cannot possibly complete, either because the enemy unit lies beyond the chargers' maximum possible charge range, or because intervening obstructions make it impossible for the unit to make a charge move that allows it to move into contact.

Note that if such an obstruction is another unit, and there is a chance that the intervening unit will move out of the chargers' way before the charge is completed, the charge is 'possible', and therefore can be declared (remember that you still need to see the target to declare the charge though).