ON-LINE MONITORING OF TOOL WEAR FOR ADAPTIVE CONTROL (AC) IS BECOMING PROGRESSIVELY CRUCIAL AS WE PROCEED TOWARD THE UNMANNED FACTORY OF THE FUTURE. THIS PAPER PRESENTS THE STATE-OF-THE-ART OF THE SUBJECT. IT HIGHLIGHTS THE CHARACTERISTICS DESIRED IN SUCH SYSTEMS, THE SYSTEMS PRESENTLY IN USE, THEIR MERITS AND LIMITATIONS. THE MAJOR FINDINGS HAVE BEEN CONDENSED IN TABULAR FORM FOR FAST APPRAISAL OF THE AVAILABLE INFORMATION. FUTURE TRENDS IN TOOL WEAR MONITORING FOR AC ALSO HAVE BEEN INDICATED.