The expansion of an initially uniform gas sphere into a vacuum is described. The analysis proceeds on the basis of free molecular flow. It is demonstrated that collisions will not seriously modify the expansion process as long as the initial mean free path of a molecule within the sphere is much greater than the initial diameter of the sphere. Expressions are derived for the momentum and energy flow associated with the expansion of such a sphere. The expansion of a gas which fills half space and also of a gas initially of the form of a uniform cylinder are described. The interaction of an expanding gas cloud with a specularly reflecting surface is determined. The interaction of an expanding gas cloud with a diffusely reflecting surface is considered.