Synthetic biology lies on the interface between natural and artificial life. It consists of the assembly of natural biological components into artificially configured biological systems. A main focus of synthetic biology has been the engineering of new gene circuits that can produce artificial cellular functions. I propose to scale up this approach to include, beyond single cells and gene circuits, also entire multi-cellular organisms and the brain circuits that regulate their behavior. Such synthetic biology in the brain will offer new ways for understanding how brain connectivity relates to brain function, and could ultimately lead to futuristic technologies such as neuronally-programmed organic robots or biologically-based brain repair. As a first step towards this ambitious goal I have developed a technique for genetically inserting new synaptic connections into the nervous system of the nematode worm C. elegans, enabling the manipulation of information flow in the nervous system and the reprogramming of whole animal behavior in this organism. This approach may be expanded and adapted to other genetic models, and opens the way to possible new forms of artificial life. Such technology, if practiced responsibly, could offer considerable benefits to science, industry and medicine.