This paper presents a seven-code system for the classification of automatic tool changers for machining centers according to their complexity. The first code is the types of tool magazines. The second code is the relative positions of the tool magazine and the spindle. The third code is the types of tool change mechanisms. The fourth code is the types of motion for grasping tools. The fifth code is the types of insertion and extraction of tools. The sixth code is the types of motion of changing tools. The last code is the positions of the spindle. Several examples are given to demonstrate the applications.