Despite the recognized importance of in vivo fatigue behavior, there is still a very limited understanding of how fatigue cracks propagate in Nitinol.  This study represents the pioneering approach of combining fracture mechanics and X-ray (micro)diffraction from synchrotron sources. These investigations provide insight into the role of transformational and local strain fields on the progression of fracture in stent-like material.   The results presented here compare the fracture-mechanics-predicted transformation zone size and shape with the actual zones measured by micro-diffraction. Tests were conducted with compact-tension specimens, laser-cut from Nitinol tube that was shape-set flat; this configuration mimics the microstructure and texture observed in Nitinol medical devices.   Fatigue cracks were grown ex situ under near-threshold conditions (DK = 3 MPaÖm) to crack-length-to-width ratio of a/W = 0.5.  Specimens were then loaded in situ with a miniature straining rig to various stress intensities and multiple fatigue cycles. Thousands of local diffraction patterns (1 mm² spot area) spanning hundreds of microns surrounding the crack tips were combined to produce contour maps of phase volume and local strain, which coupled with finite element analysis can infer the local stress fields. The differences in monotonic and cyclic loading conditions were deduced from these tests to address the differences of in vivo single-event versus cumulative-damage fractures.