Complexity theory can refer to more than one thing:

• Computational complexity theory: a field in theoretical computer science and mathematics dealing with the resources required during computation to solve a given problem
• Systems theory (or systemics or general systems theory): an interdisciplinary field including engineering, biology and philosophy that incorporates science to study large systems
• Complexity theory is sometimes used as a broad term addressing the study of complex systems, including subjects such as chaos theory, artificial life, genetic algorithms, and the topics studied by the Santa Fe Institute [1]