This course aims to provide a comprehensive introduction to ultra-low power digital circuit design. Starting from the transistor sub-threshold operation, this course covers both the circuit and system design aspects, including EKV model of MOSFET, minimum energy operation, digital logic, sub-threshold memories, and so on. This is an introductory course for research students who are interested in the cutting-edge digital circuit design methodologies and eventually improving the understating of digital circuit design.