This is a review of closed loop natural circulation loops with prescribed heat flux conditions over its length. The one-dimensional mathematical model of flow and convection in the loop is first described. Multiple steady states are found as well as time-dependent chaotic behavior. The effect of inclusion of power-law friction and axial conduction on the mathematical model is also considered. There is further discussion of multiple loops, one with fluid interchange but separate heating, and the other with separate loops and heat exchange.