Missing neuronal communication between parts of the body and the brain can cause organ failures or life-threatening conditions including cardiovascular and neurological diseases. The implantable brain–computer interface (BCI) is an emerging interdisciplinary technology that can bridge the communication gap, restoring organ functions and treating neurological disorders. Since the success of the first battery-powered pacemaker in 1958, bioelectronics technology has made a prodigious leap toward a wide range of biomedical applications such as therapeutics, diagnostics, and assistive organ implants. The latest developments in material sciences and device integrations have enabled a technological paradigm shift from rigid to soft and mechanically conformal implantable BCI devices that could eliminate device–tissue mismatches. However, achieving mechanical and electrical stability with organic- and nanomaterial-based electronics implanted in the body is a big challenge. Recent advances in inorganic and wide bandgap materials for BCI devices provide a promising route to solve this bottleneck due to their stability, modalities, and standardized manufacturing processes. Nevertheless, several remaining key obstacles still hinder the applications of soft BCI devices. This review summarizes the latest developments and key challenges of this emerging field, focusing on technological and scientific aspects such as materials, device structures, modulation, sensing, power, and communication. Current challenges and future perspectives of this emerging technology will also be discussed.