The Müller AO Classification of fractures is a system for classifying bone fractures initially published in 1987 by the AO Foundation as a method of categorizing injuries according to therognosis of the patient's anatomical and functional outcome. "AO" is an initialism for the German "Arbeitsgemeinschaft für Osteosynthesefragen", the predecessor of the AO Foundation.

It is one of the few complete fracture classification systems to remain in use today after validation.

The English language version of the system allows consistent in detail description of a fracture in defined terminology by creating a 5-element alphanumeric code:

First, each fracture is given 2 numbers to describe which bone it affects, and where in the bone:

Each fracture is next given a letter (A, B or C) to describe the joint involvement of the fracture:

The exceptions to this step include:

Finally, the fracture is given 2 further numbers to denote the fracture pattern and geometry.

For segment 2 (diaphyseal) fractures: