Let O be a maximal order in an indefinite quaternion algebra of discriminant D over Q and Q_D be the set of points in Siegel’s modular threefold A_2 whose corresponding abelian surfaces have quaternionic multiplication by O. In this thesis, we first determine the number of irreducibe components in Q_D and then for each irreducible component of genus 0, we will find a parameterization in terms of the Hauptmodul of a certain Shimura curve associated to O.