Using high angular resolution diffusion-weighted images, spherical deconvolution enables multiple white matter fiber populations to be resolved within a single voxel by computing the fiber orientation distribution (FOD). Higher order information provided by FODs could improve several methods for investigating population differences in white matter, including image registration, voxel-based analysis, atlas-based segmentation and labeling, and group average fiber tractography. All of these methods require spatial normalization of FODs. In this article, a novel method to reorient the FOD is presented, which is an important step required for FOD spatial normalization. The proposed method was assessed using both qualitative and quantitative experiments, with numerical simulations and in vivo human data. Results demonstrate that the proposed method improves FOD reorientation accuracy, removes undesired artefacts, and decreases computation time compared to a previous approach. The utility of the proposed method is illustrated by nonlinear FOD spatial normalization of 10 human subjects. Key words: fiber orientation distribution; reorientation; normalization; diffusion MRI Diffusion-weighted imaging provides a unique method to investigate the architecture of white matter in vivo. To investigate group differences in the diffusion properties of white matter, correspondence between images is required to ensure that the same anatomical structures are being compared. The most common method for obtaining correspondence involves transforming data from each individual subject into a common co-ordinate or template space (a process called spatial normalization). For the remainder of this text, we refer to spatial normalization as simply normalization.The diffusion of water within each imaging voxel is commonly modeled using a diffusion tensor (1). In recent years, normalization of diffusion tensor measures such as fractional anisotropy (FA) (2) has been widely used to investigate voxelwise differences between diseased and healthy populations (see Assaf and Pasternak (3) for a recent review). However, the popular rank-2 diffusion tensor is only capable of modeling a single fiber population within each voxel. This is a serious limitation given recent work suggesting that 90% of white matter voxels contain multiple fiber populations (4). Consequently, in the majority of white matter, population differences in diffusion tensor measures can be influenced not only by differences in so-called white matter "integrity", but also by partial volume effects within crossing fiber regions (5,6).Using high angular resolution diffusion-weighted imaging, higher order models enable crossing fibers to be resolved and, therefore, provide more information than the diffusion tensor at a subvoxel level. Spherical deconvolution is one method that can resolve crossing fibers by estimating the so-called fiber orientation distribution (FOD), a continuous distribution representing the partial volume of the underlying fibers as a function of orientation (7-12...