The structure of the hypothalamus as a behavior control device is analyzed using the evolutionary-behavioral approach. This approach postulates that in order to create an algorithmic model of the structure of the brain on the n-th step of evolution, two sequences of models must be built in parallel: the models of behavior on the k-th step of the evolution and the model of the controlling this behavior device that is the model of k-th step’s of the brain evolution, where k = 1,2,…, n. Each new step in the evolution of the brain adds a new module to the control device, one that allows or prohibits the action controlled by the previous structure, depending on the inputs added at this step of evolution. The precursor of the hypothalamus, which is responsible for controlling the autonomic nervous system, is the evolutionary precursor of the parabrachial nucleus. Others deal with the olfaction-related options and the sleep-wake switching. In all of the realms the corresponding nucleus of the hypothalamus operates as a mode on/off switch.