All our brain functions, such as basic condition-controlling, learning, decision-making, are based on corresponding neural circuits, and accordingly, “Mental illness is defined as disruption in neural circuits” (the National Institute of Mental Health, NIH, 2011). For a complete elucidation of the mechanisms of a given brain function, we must know the exact structures of all circuits related to that function precisely. However, the studies at circuit level on both structures and functions have been hindered by lacking of effective tools. In the past decade, the group has been developing tools to for neuron labelling and circuit tracing, as well as function analyzing. By using molecular, genetic, and cellular methods to modify and construct viral tools, hundreds of viral tracing systems have been successfully established and used in hundreds of laboratories and hospitals for a variety of purposes in non-human primate, rat, mouse, and zebra fish. I will give a brief update of the progresses in our laboratory on the relevant topics, including some applications.