A central challenge  of the 21st Century is to understand the human brain, not only because we are  fascinated by its incredible capabilities but also because increasingly,  neuropsychiatric disorders (mental and behavioral disorders plus neurological  diseases) dominate the health care burden in all industrialized societies. This  challenge has led to major initiatives such the Human Connectome Project and the  “Obama” BRAIN Initiative in the US, and the Human Brain Project in Europe.  The  US BRAIN Initiative identifies “the analysis of circuits of interacting neurons  as being particularly rich in opportunity, with potential for revolutionary  advances” [1], and focuses on  “accelerating the development of technologies for mapping the circuits of the  brain” [1]. In this effort,  non-invasive magnetic resonance imaging techniques have evolved as the dominant  methodology for the study of human brain, going from early experiments  demonstrating relatively course images of activity in the visual cortex to  providing detailed information on functional and anatomical connectivity in the  mm scale, and mapping the activity of sub-millimeter neuronal ensembles that  represent elementary computational units, such as cortical columns with laminar  resolution. These non-invasive approaches are complemented by equally promising,  photon based neuroimaging techniques for mapping activity in animal models in  the meso scale, albeit only in animal models due to their invasive nature.  Together these approaches span multiple scales in the intricate organization  that underlie brain function, and promise major advances towards understating of  the human brain activity in health and disease.