This chapter presents attitude synchronization schemes for a group of rigid-body systems. The proposed approach is based on the introduction of unit-quaternion-based auxiliary systems to remove the requirement of angular velocity measurements. Several control problems are addressed, namely the leaderless/leader-follower attitude synchronization and cooperative attitude tracking problems. Moreover, a conceptually similar approach is considered to design velocity-free attitude synchronization schemes that account for input saturation constraints. The information flow between spacecraft is assumed to be fixed and undirected.