Neural circuits are composed of a meshwork of numerous neuron types, each with their own distinctive morphological and intrinsic physiological properties, connectivity and biochemistry. How do distinct neural subcircuits, composed of different classes of neuron, contribute to brain function? Approaching this question requires methods that can target specific neurons types. This can be achieved by harnessing the same machinery that builds sophistication into the brain and using it to make novel tools for investigating and manipulating the brain: molecular and genetic technology. These tools can be used to target gene expression to specific neuron types within complicated neuronal circuits, and the transgenes that are expressed can be used to elucidate and manipulate these circuits with unprecedented precision and control. These methods are likely to become the archetype for future studies linking perception, cognition and behavior to specific components of the brain.