The Charge Beam is a beam upgrade that has been featured in every game since Super Metroid.
The Charge Beam has remained integral in each game it has appeared in since its debut, as it allows its user to charge up its shot to fire a more powerful version of the beam currently in use. A charged shot can do damage to bosses, something an uncharged shot can't do in the 2-D Metroid titles.
In the 3-D Metroid titles the Charge Beam isn't necessary to do damage to bosses but it does help, especially when Missiles are scarce!
 Super Metroid
The Charge Beam is located in Brinstar and is generally the first beam you find. Stronger than Missiles in terms of damage output, this will make it easier to defeat stronger enemies and do damage to bosses.
If Samus drops into Morph Ball mode with a fully charged Charge Beam, she'll drop 5 Bombs instead: two on each side of her and one where Samus entered Morph Ball mode.
Samus can also do a special attack using a Power Bomb, which consists of charging up your beam with either the Ice Beam, Wave Beam, Spazer Beam or Plasma Beam turned on by themselves, then charging the Charge Beam with Power Bombs active. This creates a powerful attack that will have a different effect depending on the beam that is equipped!
This would essentially be the beginning of Charge Combos in the Metroid franchise.
 Metroid: Fusion
The Charge Beam function is restored in Sector 1, hidden by a Core-X that has disguised itself as a Chozo Statue. Shooting the orb that the statue is holding starts the fight, but here's the catch: you can't damage the Core-X unless it opens up an eye-like hatch. Shoot a Missile into that to do damage; absorbing the Core-X after the battle restores the ability.
 Metroid: Zero Mission
Despite being absent in the first Metroid game, the Charge Beam can be found early in the game. It is held by a worm mini-boss that'll run to another room if you don't defeat it quickly enough! Shoot Missiles into its open eye to take it out and claim the Charge Beam!
 Metroid Prime
Samus starts the game with the Charge Beam, but it malfunctions in the intro. To get the Charge Beam, head to the Watery Hall in the Chozo Ruins. In this chamber, it is necessary to scan the four Runic Symbols to open the final lock on a security gate, which will open when it is scanned. The Charge Beam will make life a lot easier in this adventure, especially with Charge Combos that can only be performed with the Charge Beam.
Samus starts Metroid Prime 2: Echoes and Metroid Prime 3: Corruption with the Charge Beam and doesn't have to find the upgrade in these games. Also, Samus will continue to use the Charge Beam in Metroid: Other M regardless of Adam Malkovich authorizing its use.
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