Reputation Grinding Guide (WoW Wiki)

Reputation is very similar to experience. Reputation is divided into a number of different levels for which players must earn reputation points to progress through. Higher reputation levels generally require more points than the previous level to progress. The exception is hated, which spans a very large number of reputation points. Reputation points, just like experience points, are gained through quests and killing various mobs. Unlike experience, it is possible to lose reputation points with some factions, either by killing members of the faction or by assisting rival factions. For rival factions, gaining reputation with one will normally decrease reputation with the other, but at a greater rate. For instance, if you kill a magram centaur, you will gain 20 points of reputation from the gelkis, but will lose 100 points from the magram. Currently there are 4 of these rival-faction pairs: Aldor (Draenei) vs. Scryers (Blood Elves), Gelkis vs. Magram (both Centaur), Steamwheedle Cartel (Goblins) vs. Bloodsail Buccaneers (pirates), and Ravenholdt (rogues) vs. Syndicate (Thugs).

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