Intra-aortic balloon counterpulsation is the most widely used therapy for support of a compromised left ventricle. The principles of counterpulsation were developed in the 1950s, and intra-aortic balloon pumps have been used for more than 40 years. Despite this long-standing clinical use, many of the timing practices have continued almost unchanged from their inception. One of the most important aspects of the pumps is timing, or synchronizing the action of the device with the cardiac cycle. The principles of timing are based on the physiological objectives of counterpulsation; however, research into alternative timing methods has led to conflicting and often confusing information on the appropriate timing method for a specific clinical situation or patient. Although a body of knowledge is available, much of the research is dated and covers only specific timing methods or populations of patients. Further evidence is needed to support the selection of timing methods and determine the clinical benefits of the various methods.