From an engineering point of view, the nervous system, which is made of neural tissue, is a most complex biological system. All possible instruments or tools for applications related to the nervous system are part of another type of complex systems: biomedical instrument systems. As in any interacting system, “communication” can occur at the interface of these two systems. Depending on the kind of application, this communication can be either unidirectional or bi-directional. The signals found in these two systems are of very different nature.