Given that the sun is the initiating contributor to the Earth’s climate (Chap.  1 ), we may surmise that with the longest day and highest sun angle occurring at the summer solstice (June 20th or 21st in the northern hemisphere) that this would be the time of maximum ocean heating. However, the rates of ocean heating or cooling, by virtue of the net exchange of heat across the atmosphere–ocean interface, are somewhat more complex. The surface heat flux (or the flow of heat energy per unit area per unit time, as expressed in units of Joulesm−2 s−1 or Wattsm−2) depends not only on solar radiation, but also on the radiation that emanates from the ocean surface itself, plus that from clouds, greenhouse gases found in the atmosphere and the turbulent exchanges of heat across the atmosphere–ocean interface. This chapter explores these radiative and turbulent heat fluxes and how these lead to a seasonal cycle in the upper ocean’s temperature.