NRRD (Nearly Raw Raster Data) is a file format and software library designed to support N-dimensional raster data, which is frequently used in scientific visualization and medical imaging. Developed as part of the Teem toolkit, the format was created to provide a simple, flexible alternative to more complex formats like DICOM or HDF5. A NRRD file typically consists of a human-readable ASCII header followed by the binary data, though the header can also be stored in a separate file using the .nhdr extension. The header contains critical metadata such as the number of dimensions, data type, encoding, and spatial orientation information, which is vital for interpreting volumetric data from sources like CT or MRI scans. Because it supports various encodings including raw, gzip, and bzip2, it is highly efficient for storing large datasets while maintaining ease of use for researchers and developers. Its ability to represent complex spatial transformations and coordinate systems makes it a standard choice in neuroimaging and diffusion tensor imaging (DTI) research.