This chapter proposes a classification of surgical assistance systems with respect to their type and level of automation. This classification is based on previous work in the field of human factors and takes two aspects into consideration, the type of information-processing function of the surgeon that is supported by the system, as well as the type of function allocation between surgeon and systems. With respect to the former, three basic functions are distinguished, referred to as information acquisition and analysis, decision making and planning, and execution of surgical action. With respect to the type of function allocation, the status of being either "passive" or "active" comes into consideration for both objects of reference (i.e. the surgeon and the machine), depending on whether a given function is mainly performed by the surgeon, by the system, or collaboratively by both. Hence, a classification results for intraoperative assistance systems in six categories, each of these representing a different degree of automation. The classification scheme is explained and illustrated on the basis of examples of surgical assistance systems from various fields.