Conceptual models are a vital tool for understanding the processes that maintain the global ocean circulation, both in nature and in complex numerical ocean models. In this chapter we provide a broad overview of our conceptual understanding of the wind-driven circulation, the thermohaline circulation, and their transient behavior. While our conceptual understanding of the time-mean wind-driven circulation is now fairly mature, basic questions remain regarding the thermohaline circulation, for example, surrounding its overall strength and stability. Similarly, basic questions remain regarding the transient adjustment and internal variability of the ocean circulation.