BECAUSE DISTURBANCES IN MACHINING OPERATIONS CANNOT BE COMPLETELY ELIMINATED, IT IS IMPORTANT TO DETECT THEM WITH THE HELP OF MONITORING SYSTEMS; HOWEVER, IS SOME CASES THE RELIABILITY OF THESE SYSTEMS IS NOT SUFFICIENT. DIFFERENT APPROACHES WERE MADE TO OVERCOME THESE PROBLEMS. ONE POSSIBILITY IS THE USE OF A WORKPIECE MODEL TO ALLOW FOR ON-LINE CALCULATION OF CUTTING FORCES. ANOTHER APPROACH IS THE COMBINATION OF DIFFERENT SENSOR SIGNALS LIKE CUTTING FORCE AND ACOUSTIC EMISSION, WHICH CAN ELEGANTLY BE DONE BY A NEURAL NETWORK. THESE METHODS ARE PRESENTED TOGETHER WITH EXAMPLES FROM TURNING, MILLING AND DRILLING OPERATIONS.