The review focuses on sensitivity in ultrasonic flaw detection in rails and related accompanying issues. The set of topics considered includes changes in definitions of terms, used measures and methods of sensitivity adjustment that have occurred over the past 25 years. Amplitude characteristics of detected defect are considered in detail. The article is supplemented with practical examples of sensitivity settings, as well as measurements of signal amplitudes and defect detection coefficient for various cases. Keywords: ultrasonic inspection, rail testing, sensitivity, amplitude characteristics, defect detection coefficient.