A full-field thermoelastic technique for measuring stress intensity factors in dynamically loaded fatigue crack specimens is outlined. Thermoelastic Stress Analysis (TSA) uses an infrared focal plane array detector and high-speed processing electronics to produce stress images of cyclically loaded specimens. The TSA data is then used in combination with a fitting algorithm to determine stress intensity factors for mode I and mixed mode I and II crack geometries. The technique is non-contacting, fast and accurate, and has the potential to greatly enhance the efficiency of fatigue and fracture testing. An analytical solution for the complete stress state in terms of the first stress invariant is derived in support of the full-field TSA technique. Results using the technique on mode I and mixed mode specimens are presented, and show good agreement with accepted values.