Neurosystems engineering (NE) is a new interdisciplinary field linking neuroscience and engineering that focuses on the development and organization of complex systems linking behavior, cognition and technology. The need for NE arose with the increase in complexity of both neuroscience and engineering, as well as the rapid advances in the underlying science and technology and the realization that our nation's most important problems are at the nexus of neuroscience and engineering.  As an approach, NE is: 1) holistic and interdisciplinary, 2) focused on defining customer needs and functionality early in the development cycle, and 3) iterative in considering the complete problem in design synthesis and system validation necessary to document ongoing requirements and successful implementation.  Taking an interdisciplinary approach to NE is inherently complex, since the behavior of and interaction among system components are not well defined or fully understood.  NE encourages use of personnel (e.g., cognitive psychologists, anthropologists, neuroscientists, engineers) tools (e.g., brain imaging, modeling and simulation, high performance computing) and techniques (e.g., TMS to stimulate response, independent component analysis of neuronal networks) to better comprehend and manage complexity in linking brain, behavior, and technology.