Therapeutic ultrasound is one of the most widely used electrophysical agents. The users are generally not well aware of the details of the technology of ultrasonic generators and cannot be asked to perform the complicated physical measurements to test their equipment. Frequently, the only way to check the performance of the device is the observance of oil or water bubbles on the surface of the piezoelectiric probe. If bubbles are present, the generator is regarded as operating correctly. Many surveys on the output of therapeutic ultrasound equipment have revealed discrepancies between the indicated and actual ultrasonic output of many devices. International safety standards recommend a limit below 30% variation in acoustic intensity for ultrasound therapy equipment. So, further improvements in the accuracy of ultrasound machine calibration are needed. Proper calibration is essential to provide patients with a more accurate ultrasound dosage and, therefore, with a safer and more appropriate treatment. This review highlights the need for users to be aware of the potential inaccuracy of the machine and of the importance of regular calibration. They could be informed better with a systematic training in their education curricula on specific variables of ultrasound machines. Turk J Phys Med Rehab 2011;57:94-100.