Over the course of the past two decades, observational surveys have unveiled the intricate orbital structure of the Kuiper Belt, a field of icy bodies orbiting the Sun beyond Neptune. In addition to a host of readily-predictable orbital behavior, the emerging census of trans-Neptunian objects appears to display dynamical phenomena that cannot be explained by interactions with the known eight-planet Solar System alone. Specifically, the observed physical clustering of orbits with semi-major axes in excess of ∼ 250 AU, the detachment of perihelia of select Kuiper belt objects from Neptune, as well as the dynamical origin of highly inclined/retrograde long-period orbits remain elusive within the context of the classical view of the Solar System. This newly outlined dynamical architecture of the distant solar system points to the existence of planet with mass M9 ∼ 5M⊕ on a moderately inclined orbit with a semi-major axis a9 ∼ 400−800 AU and eccentricity e9 ∼ 0.4−0.6. In this talk, I will review the observational motivation, dynamical constraints, and prospects for detection of this proposed object known as Planet Nine.