EMP Shockwave (or commonly EMP) is an offensive spell cast by the Terran Science Vessel. It fires a special missile that flies to the targeted point and explodes, releasing a powerful electro-magnetic pulse. Any unit or building within a 3-matrix radius of the blast point will have its shields and energy reduced to zero. Consequently, EMP is particularly useful in the TvP match-up.
The animation for EMP is larger than the actual area of effect. EMP is effective within a 3-matrix radius of the spell's impact. In the image, the boxed Pylons would lose their shields if the middle Pylon is targeted.
After a transition is made into Battlecruisers, Science Vessels, equipped with the EMP ability, are often deployed to reduce the opponent's Battlecruiser's energy, thus denying the use of the Yamato Cannon. Alternatively, if a player relies heavily on Wraiths, EMP may be used to deny Cloaking or, if the Wraiths are his own, be used on the opponent's Comsat Station to deny scanning.
Late game Protoss is equipped with powerful spells in Recall and Stasis Field. EMP is often used to prevent the opponent from being able to use them. When threatened with an imminent Recall, Science Vessels are often seen patrolling the Terran base in hopes of EMP-sniping an incoming Arbiter. When fearing Stasis Field, Vessels are sent out to the Protoss army, trying to get an EMP off on the Arbiters. EMP is generally not found to be effective as a mere shield-reducing weapon versus the Protoss army as the splash radius is too small and the energy is better used on Arbiters. (An exception to this would be a case where the enemy has a large number of Archons as 97% of the Archon's massive health total is shields.) However, EMP is used in this manner versus high-priority units like Carriers, whose clumping makes the spell all the more effective. Famously, EMP can be used to dispel the shields of a Nexus, enabling a single Nuke to take it out.
EMP is very rarely used fighting Zerg as Irradiate is usually the better investment. EMP can be deployed to deny spells instantly (whereas Irradiate takes time) but victim Defilers can use Consume to regain that same energy almost as fast. For discussion, see TvZ Vessels EMP.
 In Combination with Other Spells
- Stasised units are unaffected by EMP.
- EMP will dispel Hallucinations.
- EMP will affect units in Maelstrom or Lockdown.
- Defensive Matrix will not prevent the loss of shield or energy by EMP.
- Broodling energy will not be dispelled by EMP.
- Science Vessels can't EMP themselves, but EMP will affect own and allied units.
- EMP deals both air-to-ground and air-to-air splash damage.
- A Shield Battery will lose both shields and energy from EMP.