During the last two decades the development of automatic methods for modulation classification has been an ongoing effort. The development has been driven by the need to replace human operators with automatic systems to be able to cope with the dense electromagnetic environment encountered in signal surveillance situations. A second factor that has had a great influence on the development is the advance in the area of signal processing, both in terms of theoretical advances and hardware improvements. This report will give an introduction to the area of modulation classification. The material is not focused on any specific modulation classifier, but will give a general introduction to the area. The methods presented in the report are most suited for older modems with data rates up to 1200 Bd.