This chapter focuses on the attitude synchronization problem of multiple rigid-body systems in the presence of communication delays. First, attitude synchronization schemes relying on the availability of the full state vector are presented. In particular, solutions to the leaderless/leader-follower attitude synchronization and the cooperative attitude tracking problems are proposed in the case of time-varying communication delays and undirected communication topology. Then, a virtual systems based approach is proposed to design synchronization schemes that solve the above problems and remove the requirement of angular velocity measurements. It is shown that this virtual-systems-based approach can be used to handle constant communication delays under a directed communication topology.