This dissertation focuses on the six-DOF control problem of spacecraft proximity operations. Sliding mode control (SMC), as a robust control method, is chosen to handle external disturbances. Dual quaternion parameterization, which can represent the translation and rotation of a rigid body simultaneously, is used to model the coupled relative motions during proximity operations. The basic idea and the key strategy of this dissertation is to explore the similarity between attitude control based on quaternions and pose control based on dual quaternions. Based on this basic idea, this dissertation first seeks to make new developments in SMC for spacecraft attitude maneuvers using unit quaternion parameterization. To be specific, this dissertation seeks to have a unified sliding surface design for spacecraft attitude maneuvers with/without attitude constraints. In the next step, this dissertation extends these developments to the relative pose control of spacecraft based on dual quaternions with/without motion constraints during proximity operations.