Restoring damaged brain function using patient-derived neurons is no longer a distant dream, but many hurdles and questions remain. What are the optimal conditions under which integration can happen? Using our recently developed xenotransplantation technique, we study how juvenile human pluripotent stem cell derived cortical pyramidal neurons integrate in the adult mouse brain. We show that human transplanted cortical neurons develop at their own intrinsic pace; nevertheless they integrate robustly and gain detailed sensory responses. Currently, we are exploring the rules of connectivity of transplanted cortical neurons, and whether one can enhance their circuit integration and functional maturation using optogenetics.