The purpose of monitoring anesthesia is to assess the plane of anesthesia for its suitability for the procedure, to evaluate the animal's physiologic response to anesthetic agents in order to recognize impending problems and to undertake pre-emptive intervention so as to result in an uneventful recovery. The purpose of this lecture is to give an overview of a number of the anesthetic monitoring devices available today. Each type of monitor gives information regarding a different physiologic system. Some monitors address ventilatory function, some address oxygenation, some address blood pressure, some address cardiac rhythm. There are multiparameter monitors and single parameter units. Is there one that is better than another in terms of patient safety when it comes to anesthesia? What does the information really mean? We will address a number of these questions and try to clarify the true value of anesthetic monitoring and perhaps help the individual practitioner make some choices about what anesthetic monitoring techniques and equipment would be most useful in his or her practice situation.