The main goal of this chapter is to give characteristics, evaluation methodologies, and research examples of collaborative augmented reality (AR) systems from a perspective of human-to-human communication. Section 2 introduces classifications of conventional and 3D collaborative systems as well as typical characteristics and application examples of collaborative AR systems. Section 3 discusses design considerations of collaborative AR systems from a perspective of human communication. Section 4 discusses evaluation methodologies of human communication behaviors. Section 5 discusses a variety of collaborative AR systems with regard to display devices used. Finally, Section 6 gives conclusion with future directions. This chapter will be a good start point to learn existing collaborative AR systems, their advantages and limitations. This chapter will also contribute to the selection of appropriate hardware configurations, software designs of a collaborative AR system for given conditions.