BrainWeb is an online interface that provides a 3D simulated brain MRI database for the neuroimaging community. It addresses the need for validating and evaluating the impact of image acquisition parameters on brain mapping and analysis techniques. The database includes realistic simulated brain MR image volumes generated using an MRI simulator that varies specific imaging parameters and artifacts. The simulator starts from a fuzzy digital phantom representing different tissue types and computes the MR signal using the Bloch equations, accounting for partial volume effects and image noise. The database covers two anatomical models (normal and with moderate MS lesions) and three imaging sequences (T1, T2, PD) with various slice thicknesses, modes, and intensity non-uniformity (INU) levels. Users can also run custom MRI simulations on the server with different pulse sequences, source phantoms, and acquisition artifact values. The 3D image volumes are available for interactive exploration and download in stereotaxic space through a WWW browser. BrainWeb aims to facilitate research and education in neuroimaging by providing a comprehensive and controlled environment for testing and validating imaging techniques.BrainWeb is an online interface that provides a 3D simulated brain MRI database for the neuroimaging community. It addresses the need for validating and evaluating the impact of image acquisition parameters on brain mapping and analysis techniques. The database includes realistic simulated brain MR image volumes generated using an MRI simulator that varies specific imaging parameters and artifacts. The simulator starts from a fuzzy digital phantom representing different tissue types and computes the MR signal using the Bloch equations, accounting for partial volume effects and image noise. The database covers two anatomical models (normal and with moderate MS lesions) and three imaging sequences (T1, T2, PD) with various slice thicknesses, modes, and intensity non-uniformity (INU) levels. Users can also run custom MRI simulations on the server with different pulse sequences, source phantoms, and acquisition artifact values. The 3D image volumes are available for interactive exploration and download in stereotaxic space through a WWW browser. BrainWeb aims to facilitate research and education in neuroimaging by providing a comprehensive and controlled environment for testing and validating imaging techniques.