The quest to understand consciousness, once the purview of philosophers and theologians, is now actively pursued by scientists of many stripes. In this talk, we discuss consciousness from the perspective of theoretical computer science (TCS), a branch of mathematics concerned with understanding the underlying principles of computation and complexity, especially the implications of resource limitations. In the manner of TCS, we formalize the Global Workspace Theory (GWT) originated by cognitive neuroscientist Bernard Baars and further developed by him, Stanislas Dehaene, and others. Our principal contribution lies in the precise formal definition of a Conscious Turing Machine (CTM). We define the CTM in the spirit of Alan Turing’s simple yet powerful definition of a computer, the Turing Machine (TM). We are not looking for a complex model of the brain nor of cognition but for a simple model of (the admittedly complex concept of) consciousness. After defining CTM, we give a formal definition of consciousness in CTM. We then suggest why the CTM has the feeling of consciousness. The perspective given here provides a simple formal framework to employ tools from computational complexity theory and machine learning to further the understanding of consciousness.