FreeSurfer is a comprehensive suite of tools for analyzing neuroimaging data, providing algorithms to quantify the functional, connectional, and structural properties of the human brain. Initially developed to generate surface representations of the cerebral cortex, it has evolved to automatically create models of most macroscopically visible structures in the brain from any reasonable T1-weighted input image. The software is freely available, runs on various hardware and software platforms, and is open source. Key functionalities include volumetric segmentation of brain structures, hippocampal subfield segmentation, inter-subject alignment based on cortical folding patterns, white matter fascicle segmentation using diffusion MRI, parcellation of cortical folding patterns, estimation of architectonic boundaries, mapping of cortical thickness, and construction of surface models of the cerebral cortex. The development of FreeSurfer was motivated by the need for accurate and topologically correct surface models, which were initially used for EEG/MEG source localization. The software has since expanded to include advanced features such as whole-brain segmentation, probabilistic estimation of cytoarchitectonic boundaries, automated tractography, and longitudinal analysis. FreeSurfer has been widely used to improve understanding of neurological disorders, genetic influences on brain structure, and healthy development and aging.FreeSurfer is a comprehensive suite of tools for analyzing neuroimaging data, providing algorithms to quantify the functional, connectional, and structural properties of the human brain. Initially developed to generate surface representations of the cerebral cortex, it has evolved to automatically create models of most macroscopically visible structures in the brain from any reasonable T1-weighted input image. The software is freely available, runs on various hardware and software platforms, and is open source. Key functionalities include volumetric segmentation of brain structures, hippocampal subfield segmentation, inter-subject alignment based on cortical folding patterns, white matter fascicle segmentation using diffusion MRI, parcellation of cortical folding patterns, estimation of architectonic boundaries, mapping of cortical thickness, and construction of surface models of the cerebral cortex. The development of FreeSurfer was motivated by the need for accurate and topologically correct surface models, which were initially used for EEG/MEG source localization. The software has since expanded to include advanced features such as whole-brain segmentation, probabilistic estimation of cytoarchitectonic boundaries, automated tractography, and longitudinal analysis. FreeSurfer has been widely used to improve understanding of neurological disorders, genetic influences on brain structure, and healthy development and aging.