Two of the biggest mysteries in physics are the strong CP problem in particle physics and the unknown nature of dark matter. Despite its successes, the SM cannot explain either of these and thus new physics is required. In particular, a new pseudoscalar boson called the axion has the potential to resolve both mysteries. This chapter introduces and motivates the significance of the strong CP problem and dark matter, describes how the axion can solve each, and discusses how the axion can be detected. The detection portion primarily focuses on the Axion Dark Matter eXperiment (ADMX). The axion is a well-motivated solution to the dark matter and strong CP problem. The evidence for the axions existence as a fundamental particle and as dark matter, and methods for detection are discussed.