Grenade (Frag)

Fragmentation Grenade is an explosive thrown weapon that appears in all Fallout games.

Fallout and Fallout 2
Fallout and Fallout 2 Standard Fragmentation Grenade. Most effective when used on groups. 

Fallout Tactics
Fallout Tactics Standard Fragmentation Grenade. Most effective when used on groups.

Fallout 3
The Frag Grenade is the first type of grenade you'll find in the game (check the first blue Mailbox as you come into Springvale from Vault 101) and is commonly carried and used by melee equipped Raiders and Super Mutants. While it's unlikely to kill you by itself it can do a lot of damage in one hit and has a high chance to cripple arms and legs. It's blast radius is small compared to grenades you'll get access to later (such as the Nuka Grenade or Plasma Grenade) but it can still be useful against multiple attackers if they're bunched together at a pinch point like a stairwell or doorway.

The in game physics for grenades are quite realistic which means throwing "up" a stairwell or hill means they'll most likely just coming rolling back before detonating next to you. The plus side is you can throw them through doorways or bounce them around corners to attack enemies you can't see. Be careful trying to throw them through windows though as, if they hit anything they'll bounce back at you.

Are You John Rambo?
Demolitions Expert: If you take all 3 ranks (+60% dmg bonus) of this perk and max out your explosives skill Frag Grenades become Super Mutant Kryptonite. They hit for 162 damage, and that's enough to outright kill regular Super Mutants, the Brutes and Masters take 2 grenades each, or one with a little small arms mop-up. Its easy to engage a mutant after using grenades because the first blast always seems to cripple their legs making them easy pickings. Getting the Wired Reflexes perk is the only known way to increase V.A.T.S. grenade range as gunslinger and commando perk benefits don't seem to apply. Tap Outcast Casdin for the technology side quest if you're going to be a pull-the-pin-with-your-teeth styled Rambo character, you can buy cheap, nearly broken power armors and laser weapons from the merchants and trade them in for tons of Frags.

Defensive Strategies
You'll know a grenade has been thrown at you when you see a red circle with a pointer appear at the center of the screen. Your best bet is to run quickly away to get clear of the blast radius. Ducking behind cover isn't always reliable as the grenade can roll past your cover and detonate right next to you.

If you see an enemy with a grenade in their hand you can target the grenade in VATS just like any other weapon. If you hit the grenade it will detonate and damage whoever was holding it. Even if the grenade isn't in the enemy's hand, like if you sneak up on an opponent with grenades holstered, you can still target and shoot them while on the belt. Sadly, you do not receive xp for these types of sinister trick-shots.

Offensive Strategies
Frag Grenades have a small blast radius so anything you can do to get the enemy bunched together by leading them to doors, hallways, stairwells or other pinch points will increase their effectiveness. Just remember to be wary about trying to throw upwards.

The distance the grenade is thrown depends on how long you hold the trigger. Don't just tap it unless you want it relatively close to you. It does not appear that it can detonate in your hand, however.

Using VATS will give you a chance (apparently depending on your Explosives skill) of detonating the grenade as it hits the target rather than when it has rolled past and grenades have a relatively low AP cost for the damage they do but throwing them outside of VATS then following up in VATS with aimed shots to the weakest parts of the bodies is an effective alternative against groups.

Using a frag grenade on any enemy that can be frenzied (such as Robots, or Giant Ants) has a relatively high chance of triggering it. Fire Ants are especially susceptible to causing riots among themselves, as they have a ranged attack.

Weapons List
Granat odłamkowy