The brain continuously receives, integrates, and responds to an influx of sensory signals emerging from the internal organs. This is mediated not only through direct neuronal connections defined by the peripheral nervous system, but also endocrine, humoral, metabolic, and immune pathways. Despite being predominantly imperceptible, the complex brain-body cross-talk is essential to maintaining physiological homeostasis. Moreover, it is increasingly recognized to play a critical role in cognitive and behavioral functions as well as in disorders of the nervous system. The functional and anatomical diversity of brain-body pathways necessitates the development of multifunctional implantable neurotechnologies that can facilitate causal studies during behavior. Although ubiquitous in studies of brain function, electrical, optical, and chemical interrogation of organ-brain circuits remains a challenge. In this review, we discuss recent developments in multifunctional implantable neurotechnologies, highlighting material selection, device architectures, integration challenges, and power and data transfer approaches necessary to establish robust bioelectronic interfaces to brain and peripheral organs suitable for long-term studies of brain-body signaling.