A brief review of crack length measurement is made with respect to fatigue crack growth. Two contrasting, closed-loop, computer-automated systems are presented for obtaining fatigue crack growth data. A d-c potential difference method is described that can obtain long crack growth data down to threshold using a high frequency, resonance-type test machine. Measurements of load/back face strain data are made automatically while the test is running and are used to obtain values of crack closure load using two regression analyses. An automated method for measuring the crack depth, length, and profile of small cracks growing in holes using eddy currents is described. A resonance-type or a servohydraulic test machine for variable amplitude loading can be controlled by the system. The method can detect and measure cracks with depths in the range of about 0.1 to 2 mm and more than one crack can be present in a hole. Results are presented in terms of crack depth as functions of distance into the hole and angular position around the hole. The printout includes the presentation of crack profiles with increasing load cycles. Both systems can be used under constant, decreasing, or increasing ΔK and at constant R-ratio or controlled values of Kmax or Kmin. The achievement of a range of target events can be used to initiate subsequent action, and on-line control is maintained through a wide range of keyboard options. Examples of the fatigue crack growth output from long and short cracks using both systems are given for a 2024 aluminum alloy. Crack profiles are shown for reamed and cold-worked holes.