There are many circumstances in a game of Warhammer that can cause a unit to panic. These are listed on p.80 of the rulebook. To make a Panic test roll 2D6. If the total rolled is less than or equal to the Leadership value of the unit forced to make the test, the unit is fine. If the total rolled is greater than the Leadership value of the unit, the Panic test is failed and the unit must flee in the same way as if it had failed a Break test