Abstract: This presentation explores the theory of vector-valued harmonic functions on the open unit disk, with a primary focus on the harmonic Hardy spaces of the open unit disk. We begin by briefly reviewing the classical scalar-valued theory, emphasizing the Poisson integral representation and the existence of radial limits on the unit circle. We will then demonstrate that the classical results extend naturally to the vector-valued setting. Finally, we will establish that the harmonic Hardy space isometrically isomorphic to the space of the vector-valued (finitely additive) vector measures of bounded variation on the unit circle.