Negative absolute temperature, defined in terms of the Boltzmann factor, corresponds to an upside‐down energy pyramid. The example of the laser shows population inversion to be accompanied by the switch from absorption to emission, that is, from positive to negative dissipation, from increasing disorder throughout a system to increasing order in a subsystem. Early treatments of the negative‐temperature concept are by Onsager and by Ramsey. Ramsey’s ’’essential requirements’’ are shown to be too strong. A TS diagram for a Carnot cycle operating between two negative temperatures clarifies the origin of amplification. Some biological negative‐resistance amplifiers (nerve, muscle), in particular those involving ordered arrays of water molecules, are conjectured to have negative‐temperature subsystems.