A monster's challenge rating tells you how great a threat the monster is. An appropriately equipped and well-rested party of four adventurers should be able to defeat a monster that has a challenge rating equal to its level without suffering any deaths. For example, a party of four 3rd-level characters should find a monster with a challenge rating of 3 to be a worthy challenge, but not a deadly one.

Monsters that are significantly weaker than 1st level characters have a challenge rating lower than 1. Monsters with a challenge rating of 0 are insignificant except in large numbers; those with no effective attacks are worth no experience points, while those that have attacks are worth 10 XP each.

Some monsters present a greater challenge than even a typical 20th-level party can handle. These monsters have a challenge rating of 21 or higher and are specifically designed to test player skill.