Bayes' Theorem lies at the heart of the increasingly important field of subjective probability. This concise introduction discusses the basic idea behind the theorem; derives an equation that represents one of its simplest forms; applies the equation to problems taken from business, medicine, personal life, and public policy; and explores some interesting features of the theorem. It is written at an elementary level for non-mathematicians who have no prior training in probability or statistics. Mr. Kirkland is a graduate of Yale University and Harvard Law School.[February 2018 Note: A small number of typographical, formatting, and stylistic changes have been made in response to helpful comments made by reviewers. No substantive changes have been made to the original text.]