PK Thunder (PKサンダー, PK Sandā) is the up special move for Ness and Lucas. When used, it creates a stream of lightning whose direction can be controlled using the control stick or d-pad. As such, using PK Thunder will result in Ness and Lucas not being able to move or defend, because the control stick or d-pad is used to aim PK Thunder.
 Ness's version
The head of Ness' PK Thunder does 8% damage when hitting an opponent. Ness's PK Thunder hits once and disappears if it hits an enemy. The tail of Ness's PK Thunder is also longer, and deals more stun than the tail of Lucas's PK Thunder. Ness's PK Thunder is also considerably faster than Lucas'.
In Melee, the uncontrolled thunder can hit anybody, not just Ness' opponents. This means that one can actually use the PK Thunder as a barrier, using the tail to intentionally block oneself from flying off the screen after being hit. This is called a PK Thunder Barrier.
 Electric Ness Glitch
In Melee, Ness's PK Thunder can be used to perform the Electric Ness glitch. When Ness uses PK Thunder from the air, lands, and is eaten by Yoshi's Egg Lay, he will emerge with sparks surrounding his body.
 Lucas's version
Lucas' does 9-10% damage, compared to Ness's 8%. Lucas' PK Thunder turns in a tighter circle than Ness', meaning that Lucas' is easier to control and can be used to recover in narrower spaces. Lucas's PK Thunder goes through enemies and can score multiple hits.
Both sides of the head have a direction of knockback perpendicular to the direction the thunder is moving. The sides can hit a shield safely without the thunder disappearing, and due to hitbox renewal (the head can only hit once every 48 frames) it is possible for the thunder to go through shields. Both PK Thunders can also break Smash Balls, making it useful to acquire a Final Smash if the Smash Ball is far away. The tail for both characters' PK Thunder cannot be cancelled.
 PK Thunder 2
PK Thunder 2 or "PK Rocket" is the term used for hitting Ness or Lucas with their own PK Thunder while it is still under their control. The Japanese developers called this move PK Jibaku (PK自爆). The word jibaku means 'self-destruct' or 'suicide bomb,' a fitting term for a tactic that turns Ness and Lucas into human missiles. This technique can be used by Ness and Lucas. Doing so will transform Ness or Lucas into a human projectile, which will move in different directions depending on what part of the body is hit. If the Ness or Lucas makes contact with an enemy, he will deal a good amount of damage and knockback, although the Smash 64 version is considerably stronger than the Melee equivalent. CPUs playing as either Ness or Lucas are also programmed to use this strategy, but they only use PK Thunder 2 vertically, even in situations where only recovering horizontally or diagonally would prevent death. The PK Thunder 2 is also Ness and Lucas's main method of recovery, given that it launches them fairly far in a straight direction. While using the move, the user will fall much slower.
 Ness's version
Ness's version of PK Thunder 2 deals one strong blow. Also if used up close, it can bring Ness to a short stop. If Ness's PK Thunder 2 hits an opponent, it does 25% damage with extremely high knockback, KOing opponents at high percentage in Melee, and in Brawl, it KOs at 30-35% depending on the weight of the opponent.
Ness's PK Thunder curves in a larger arch than Lucas, so Lucas requires less room to use PK Thunder 2 than Ness. However, Ness's PK Thunder is faster. Unfortunately, using this in a pit makes it more difficult to guide the PK Thunder without striking the walls of the pit.
 Lucas's version
Lucas's version of the attack hits multiple times, compared to Ness's one strong blow. It is interesting to note that Lucas's PK Thunder 2 covers more ground than Ness's, so Lucas's is slightly more effective. If all of Lucas's hits connect when using PK Thunder 2, it will deal a massive 43% damage and has decent knockback, making it Lucas' most damage-dealing move, with the exception of PK Starstorm, which cannot be used in competitive play. However, it is very hard to get the full 43% as PK Thunder 2 takes very long to start and as such, is easily dodgeable. For tactical uses, Lucas's PK Thunder 2 can be used to "Thunderglide". Thundergliding is when the player runs off a platform then immediately uses PK Thunder. The effect should come out to being the PK Thunder being much closer to Lucas than normal. Then, guide the thunder into Lucas and he should angle down towards the ground. If performed right, Lucas's direction will go from diagonal to horizontal as he glides along the ground. Otherwise, he'll crash into the ground like a normal poorly aimed thunder. Lucas will stop at the edge on his own, making this move safe to use on a stage like Battlefield. The thunderglide is great for surprising opponents for a quick 20-43% damage. It should also be noted that on stages like Battlefield or Final Destination if Lucas's PK Thunder 2 is aimed wrong, the user will bounce right off the corner of the stage, leaving him in a helpless animation away from the ledge.
PK Thunder (PSI Thunder in North America) first appeared in Mother and has appeared in later Mother games since. In all of three of the Mother games, the move involved using the character's psychic power to send lightning at the enemy. However, in the Mother series, neither Ness nor Lucas learn the move (which is something that Paula's trophy in Melee explains). In the description for the move in EarthBound, it states that the player could miss the target and not cause any damage to the enemy. The difficulty of using PK Thunder in the Super Smash Bros. games could be a reference to how PK Thunder can sometimes miss enemies in EarthBound. The move's recovery properties are somewhat similar to Teleport α, where the party's speed is increased greatly, to the point of being difficult to control, and explode if they hit a solid object before covering sufficient ground to warp. 
 Description from the Melee instruction booklet
Strike yourself with thunder to become an electrified projectile.
 Description from the Brawl instruction booklet
Release a streak of lightning you can control with the Control Stick. Guide it into Lucas to turn his body into a deadly projectile.