Difficulty mode

A difficulty mode is a concept used in video games, including the Prime Series, and all entries of the mainline series from Metroid Fusion on. In this sense, "difficulty" does not refer to how difficult a Metroid game is in general, but rather to a setting of the game chosen by the player at the beginning of their playthrough. Difficulty levels can range from easy to very hard. Harder difficulties are often unlocked after the first playthrough has been completed. Harder difficulties generally mean that enemies take less damage and deal more, although other challenges might be posed as well.

Main Metroid series
The Japanese version of Metroid Fusion features an Easy Mode and Hard Mode. The North American version, which was released first, does not; the Easy Mode was likely added in response to complaints about the difficulty. Metroid: Zero Mission is the first 2-D Metroid to feature Normal and Hard Modes in Western versions of the game, with Hard being unlocked by completing the game once. In both games, Hard Mode also affects expansion tanks, reducing the amount that they expand Samus's arsenal. For example, Energy Tanks only provide 50 units of energy (half of their usual 100), Missile Tanks only expand Samus's Missile Gauge by 2, and Super Missile and Power Bomb Tanks only add one additional unit.

In Metroid: Other M, Hard Mode is unlocked after achieving 100% completion. Unlike other games, the Hard Mode in this game does not change enemy damage or health. Instead, it removes all expansions in the game, essentially constituting a 0% run. This means that Samus can be killed within a few hits.

In Metroid: Samus Returns, a unique difficulty mode is unlockable via the Metroid amiibo: Fusion Mode, which changes Samus's Power Suit to the Fusion Suit. This difficulty level is significantly harder than Hard Mode, which is unlockable in the base game after completing it once, as it increases enemy damage 4x what it is in Normal Mode.

Metroid: Samus Returns manual
''The current game's difficulty level. * You can change the difficulty level for a file after completing the game and selecting "New Game".''

Metroid Prime series
Metroid Prime is the first game to feature adjustable difficulty levels worldwide. Upon completion of the first playthrough on Normal Mode, Hard Mode is unlocked. Metroid Prime 2: Echoes and Metroid Prime: Federation Force continue this trend, although Normal Mode is much more difficult than it was in Prime; this was a point of criticism for the game. Hard Mode also affects some of the Campaign mission score bonuses, such as increasing the amount of points needed to be scored, or lessening the par-time. Metroid Prime 3: Corruption introduces Veteran Mode, an "intermediate" difficulty, and replaces Hard Mode with Hypermode. New Play Control! Metroid Prime and Metroid Prime 2: Dark Echoes, as well as the Metroid Prime Trilogy, apply the Veteran and Hypermode difficulties to the first two games as well. Hypermode is the same as the original Hard Mode, while Veteran is comparable to the Normal Mode of the original games. An even easier Normal Mode has been added as well.

Metroid Prime Pinball features Expert Mode, which increases the difficulty of Mini-Games from level 1 to level 2 on the first play. Metroid Prime Hunters is the only Prime game to not have an adjustable difficulty setting.