The cerebral cortex is the site of our highest mental capacities such as learning, language, and consciousness itself. It is also arguably one of the most complex biological tissues on earth. Decades of experimentation suggest that there are in fact well defined cell types and highly stereotyped synaptic connections that wire these cell types to form various types of elementary cortical circuit, which may perform that a set of basic computations. Characterization of such cortical microcircuits is essential to any theory of cortical function. An emerging view is that the formation and functional tuning of neocortical circuits is determined by both genetic instructions and epigenetic influences such as experience. It is probably the genetic programs, however, that set up a time course for the development of these circuits and put time constraints on when experience is most effective in shaping the patterns neuronal connectivity during early postnatal life, know as the critical period. Our long-term goal is to discover the molecular mechanisms underlying the formation and experience- dependent modification of neuronal connections in neocortex at the level of defined cortical circuits and identified cortical cell types. We combine mouse genetic/genomic, physiological, and imaging approaches to study the detailed cellular architecture of neocortex. We focus on the development and experience-dependent plasticity of the GAB Aergic inhibitory circuits.