Functional neuroimaging of healthy adults has enabled the mapping of brain functions and revolutionized cognitive neuroscience. Increasingly, functional neuroimaging is being used as a diagnostic and prognostic tool in the clinical setting and to view brain development. Its expanding application in the study of disease and development necessitates new, more flexible tools. The logistics of traditional functional brain scanners (for example, magnetic resonance imaging (MRI) systems) are not well suited to subjects who are in an intensive-care unit or who might otherwise require sedation for imaging, such as infants and young children. Diffuse optical imaging (DOI)—an emerging, noninvasive technique with unique portability and hemodynamic (blood circulation) contrast capabilities—can record evoked brain function in clinical environments. However, despite its unique strengths, DOI as a standard tool for functional mapping has been limited by low spatial resolution, limited depth penetration, and a lack of reliable and repeatable mapping. To address these weaknesses, there recently has been significant interest in applying tomographic optical approaches to the problem of imaging brain function.