The human brain is an exceedingly complex biological system with billions of neurons that allows us to process and respond to a constant changing physical environment. Examining the interconnectivity of each of these regions at a circuit level provides insight to brain functions that dictate cognition and action. The work from this dissertation stands to expand our current knowledge of cerebral cortical systems and introduces a heavily understudied cortical-subcortical loop network that bridges cortical network components critical for performing cognitive functions. Using viral tract-tracing and ex vivo electrophysiology methods, I found that a subcortical structure, called the claustrum, connects specific cortical network components. In doing so, I revealed the bulk of the input-to-output circuit map of the murine claustrum. These findings support a model wherein the claustrum supports cortical networks for cognitive control. Understanding the neural circuit mechanisms underlying cortical networks is critical for treating cortical network dysfunction, and resultant cognitive impairment, across a multitude of neuropsychiatric diseases.