Concerned that excessive expansion of fiduciary status will inevitably dilute fiduciary law, the author seeks a theory of fiduciary law that can establish the outer limits of fiduciary status. Theories tend to involve a trade-off between descriptive accuracy and intellectual coherence. In order to assess and maintain the descriptive accuracy of his efforts, the author employs a concentric circle or heat-map metaphor. He begins with the fiduciary powers theory developed by Paul Miller, according to which fiduciary power is a form of authority ordinarily derived from the legal personality of another. However, the author argues that the defining factor of fiduciary relationships should be such power, without regard to whether the power is discretionary. Under this theory, trustees and agents are fiduciaries categorically. Advisers generally do not have fiduciary power over their clients, and thus should not be considered categorical fiduciaries. However, in order to maintain descriptive accuracy, the author argues that certain advisory relationships, where the adviser's influence amounts to de facto power, should be accorded fiduciary status. All other relationships, including those based on solely on trust and vulnerability, should not be considered fiduciary relationships. Nevertheless, anyone who satisfies the definition of a categorical fiduciary should be considered a fiduciary to that extent.