List of complexity classes

This is a list of complexity classes in computational complexity theory.

For other computational and complexity subjects, see list of computability and complexity topics.

Many of these classes have a 'co' partner which consists of the complements of all languages in the original class.

For example, if a language L is in NP then the complement of L is in co-NP.

"The hardest problems" of a class refer to problems which belong to the class such that every other problem of that class can be reduced to it.