Atomic Layer Deposition has emerged as a powerful, and frequently preferred, deposition technology. The interest in ALD has increased considerably in the last decade due to exceptional properties of ALD. ALD can be employed for the most demanding applications due to its superior conformality, large area uniformity and atomic level accuracy in controlling film thickness and composition. An important challenge for the industrial application of ALD is the reactor that needs to be designed to allow for the fast and cost-effective growth of films. This paper gives a short review of ALD principals, the types of ALD that can be performed, reactor designs and the main areas of ALD layer applications.