When is a person free to do something? The focus of much of the recent work on this question has been the debate between those who defend freedom as non-interference (aka liberals) and those who defend freedom as non-domination (aka republicans). This book defends a new answer to this question, freedom as non-constraint. According to freedom as non-constraint, both interference and domination cause unfreedom, both human and natural constraints cause unfreedom, and both controlled and uncontrolled interference cause unfreedom. Compared to liberal and republican theories, freedom as non-constraint provides a better account of systemic and structural threats to freedom, a better picture of how market forces and governments impact freedom, and a better understanding of how the natural world constrains freedom. Freedom as non-constraint also provides a new account of the scalarity of freedom and points to the limits on our ability to measure freedom with precision.