This thesis presents original research on how to seamlessly integrate electronics with living biological systems. Jia Liu has used silicon nanowires as active sensors to investigate biological signals at the cellular level. He has also designed nanoelectronic networks into flexible, three-dimensional (3D) and macroporous architectures, which mimic the structure of tissue scaffolds for in vitro 3D integrations with synthetic tissues and in vivo implantation by means of syringe injection. Importantly, the results demonstrate 3D interpenetrations of nanoelectronic networks with neural networks, 3D mapping of tissue activity and long-term implantation with minimal immunoresponses. Further, the book discusses potential applications for pharmacological studies, brain activity mapping and nanoelectronics enabled therapies. The findings presented here have gained wide recognition, including a top research ranking by Chemical & Engineering News and being listed among Scientific American's 10 world changing ideas in 2015.