Unraveling and controlling neural processes underlying cognition, sensation, volitional movement and neurological disorders require implantable electrodes capable of decoding the activity of millions of neurons at millisecond resolution, over months to years and without inducing foreign-body reactions. Since Luigi Galvani discovery of bioelectricity, metals have been used as electrode materials. Metals, however, are dramatically inadequate to address the chemical, optical and mechanical properties of neural cells: they are stiff, unstable in physiological environment and can induce severe mechanical and neurotoxic damage to the soft tissues. Furthermore, recent advances in neuroscience methods for imaging, stimulating and inhibiting neural activity are emerging as powerful tools, but there is no available material that can simultaneously provide optical, electronic, magnetic and electrochemical properties to interface with neural microcircuits across multiple modalities. Carbon-based nanomaterials like carbon nanotubes (CNT) and graphene combine intrinsically high mass-specific surface area, electrical conductivity, mechanical strength, flexibility, low magnetic susceptibility mismatch with tissues and biological stability, which makes them uniquely suited for interfacing with neural circuits. We have engineered CNT fibers into cellular-scale size microelectrodes for chronic recording and stimulating neural activity. CNT fibers have superior electrochemical impedance and charge transfer properties compared to metal probes of the same size, which allows minimizing implant footprint without affecting the efficiency of transduction of biological ionic currents across the electrode/tissue interface. In vivo biocompatibility, effective deep-brain stimulation and stable chronic recording neural recordings demonstrate the feasibility of CNT fiber microelectrodes for high-fidelity, minimally invasive neural interfaces. To study the wiring diagrams of neural circuits and elucidate the fundamental mechanisms giving rise to behavior and disease at high spatial and temporal resolution, we have also developed transparent graphene-based optoelectronic neural interfaces for simultaneous electrophysiology and imaging. By leveraging the optical and electronic properties of graphene we have fabricated thin-film multichannel electrode arrays with >90% transparency from the UV to the near-infrared range, making them compatible with most of the conventional and fluorescence-based neuroimaging modalities. Example of applications of these optoelectronic graphene devices are chronic platforms for in vivo optical and electrical recordings of neural microcircuits involved in epilepsy generation.