Atomic layer deposition (ALD) is a thin film growth method based on sequential, self-limiting surface reactions. ALD can produce extremely conformal and atomic layer controlled film growth. The rate of ALD film growth is dependent on surface reaction efficiencies and gas dynamics. This talk will review the design of various ALD reactors with emphasis on the role of gas dynamics. The optimum pressure in viscous flow ALD reactors will be shown to be a trade-off between gas entrainment and gas interdiffusion. The most rapid rates of ALD film growth will be achieved using "synchronous modulation of flow and draw" where the reactant exposures occur under near static conditions and viscous flow is only operative during reactant purging. The talk will conclude with a review of new ALD reactors operating at atmospheric pressure.