North America provides a natural point of departure from which to study and explore the emergence and evolution of think tanks. In addition to being home to some of the world's most prominent think tanks, North America has provided the spawning ground for hundreds, if not thousands, of smaller and more specialized think tanks. The very diversity of think tanks has presented and continues to present a major stumbling block for think tank scholars. The postwar period was also a time when many think tanks emerged in Canada. The organizations received government funding in amounts that most nongovernmental think tanks could only dream about, but operated at an arm's length relationship inside the government. The rise of advocacy think tanks starting in the early 1970s fundamentally changed the appearance of the think tank landscape in the United States and Canada. Most think tanks share a common objective of shaping and molding public opinion and public policy.