The Lawson criterion is a minimum condition on the product of ion density, ni, and mean residence (confinement) time, τ, of the energetic ions within a reacting plasma if it is to be useful as a source of thermonuclear power. This criterion is a function of temperature, T, and energy recovery efficiency, η . The curve originally presented by Lawson, for a single efficiency of 1/3, is actually the lower side of a closed contour in the ni , T plane. This is readily generalizable to a cup-like surface in ni , T, η space. There is a forbidden region within which a steady state reactor cannot operate. The Lawson efficiency, η , should not be confused with the thermal efficiency of the associated energy conversion device, which may have a lower value. Examples for selected fuel cycles will be given.