This thesis is an investigation of Chisholm's three incompatibilist theories of agential freedom. After considering the faults and virtues of each, I conclude that the most defensible theory is that which includes the concept of primitive agent causation, i.e., agent causation as a basic kind of causation, not as a kind of event causation. I then defend this concept against various criticisms that have been directed at it over the years. Finally, since agent causalists in general, and Chisholm in particular, are all motivated by incompatibilist views of agential freedom, I defend this position against various compatibilist attacks. I conclude that Agent Causalism should be treated as a live option in any serious discussion of agential freedom.