The proposed challenge is the first diffusion tractography challenge to include a three-dimensional physical phantom (sub-challenge #1), which models the intricate details of brain white matter microstructure, such as crossing and bending fibers, and allows testing of an array of acquisition conditions and tractography methods intended to extract these features. Additionally, this is the first tractography challenge to include histologically derived white matter tracts (sub-challenge #2 and #3). Histology adds an extra layer of realism to both data acquisition and potential tractographic challenges –including not only the true anatomical complexity of white matter pathways, but also tracer sensitivity on the scale of microns. The histological challenges, then, are to most accurately replicate the anatomical connections as identified by the histological tracers. Together, these three sub-challenges present a unique three-dimensional opportunity to validate diffusion tractography. These are offered in combination with our industry sponsor, Synaptive Medical, as well as NIH funded research (Anderson, Landman).