[en] Neutron emission in fission is usually described in terms of two observables: the energy spectrum of emitted neutrons N(E) and the average number of neutrons emitted per fission, or average neutron multiplicity, /bar v/p. These observables are measured before the residual fission fragments decay toward the valley of β stability and are therefore referred to as the prompt neutron spectrum N(E) and the average prompt neutron multiplicity /bar v/p. They are of fundamental importance to the design of macroscopic systems that are driven by the fission reaction, such as thermal or fast reactors. It is the purpose of this paper to describe existing theoretical models for these two observables. Other observables for neutron emission in fission will not be described here due to space limitations. 12 refs., 2 figs