A #field in #ComputerScience
Complexity Theory deals with the relative difficulty of solving computational problems. It asks how efficiently, most commonly in terms of computing time and memory, we can solve any problem.
For some problems, we can even prove that we have found the most efficient algorithm anyone could ever develop. The most important problem in complexity theory, and probably in the whole of Computer Science, is the famous problem P vs NP, which is ultimately a question about the nature of really hard problems.