Publisher Summary This chapter discusses various aspects of the flow of gases which includes measured, standard, and true leak rates, change of partial pressure with time, and viscous flow rates of different gases. It is found that transitional and molecular flows are present when there is a difference in partial pressures. Two sealed packages having different internal volumes, but having the same measured leak rates are considered. The identical measured rates indicate that the rate of helium exiting the packages are the same, but the size of the leak channels is unknown. Leak rates for different gasses through the same leak channel are different due to the variation of the gas's mass and size. It is found that if the partial pressure inside the package is greater than the partial pressure outside, the measured flow rate out will decrease with time as the partial pressure of the leaking gas decreases. The decrease in the partial pressure is proportional to the conductance of the leak channel and the initial partial pressure in the package. Viscous flow can occur when there is a difference in total pressure between the gases inside a package and the package's environment. It is shown that the change in the total pressure difference, from inside to outside the package, depends upon the flow of gases in both directions.