Characterizing and understanding the limitations of diffusion MRI fiber tractographyis a prerequisite for methodological advances and innovations which will allow thesetechniques to accurately map the connections of the human brain. The so-called“crossing fiber problem”has received tremendous attention and has continuouslytriggered the community to develop novel approaches for disentangling distinctly ori-ented fiber populations. Perhaps an even greater challenge occurs when multiplewhite matter bundles converge within a single voxel, or throughout a single brainregion, and share the same parallel orientation, before diverging and continuingtowards their final cortical or sub-cortical terminations. These so-called“bottleneck”regions contribute to the ill-posed nature of the tractography process, and lead toboth false positive and false negative estimated connections. Yet, as opposed to theextent of crossing fibers, a thorough characterization of bottleneck regions has notbeen performed. The aim of this study is to quantify the prevalence of bottleneckregions. To do this, we use diffusion tractography to segment known white matterbundles of the brain, and assign each bundle to voxels they pass through and to spe-cific orientations within those voxels (i.e. fixels). We demonstrate that bottlenecksoccur in greater than 50-70% of fixels in the white matter of the human brain. We find that all projection, association, and commissural fibers contribute to, and areaffected by, this phenomenon, and show that even regions traditionally considered“single fiber voxels”often contain multiple fiber populations. Together, this studyshows that a majority of white matter presents bottlenecks for tractography whichmay lead to incorrect or erroneous estimates of brain connectivity or quantitativetractography (i.e., tractometry), and underscores the need for a paradigm shift in theprocess of tractography and bundle segmentation for studying the fiber pathways ofthe human brain.