<p>The compliance method is widely used to measure crack length during testing of fracture mechanical properties such as fracture toughness and fatigue crack growth rate. Traditionally, the compliance is obtained from the load/deflection records. We present a time domain based differential compliance method, in which the compliance is obtained directly from the vibration waveform of a simple resonance assembly. The compliance computation from time domain signal offers high resolution and low noise providing unprecedented possibilities such as so called “rate-control” testing with fatigue frack growth rate directly controlled by a closed loop system. The differential approach enables to significantly reduce the effect of specimen clamping and material property changes during the test. The method has been utilized in many research project and its gradual development was described in several papers. This, paper, however, for the first time, summarizes and updates all important details of the technique necessary for its implementation as well as the derivation of the used vibrational model. It also describes the advantages and disadvantages of the method and its application potential in testing materials resistance to fatigue crack growth.</p>