Atomic layer deposition (ALD) is under active research for its many emerging applications. In the optical field, ALD opens fundamentally new research paths, providing extreme conformality and capacity to engineer new materials. ALD enables coatings which are undoable or difficult through physical vapor deposition (PVD), like films inside tubes, highly repeatable sub-nanometer thick films, double-sided deposition, large-area accurate coatings, and three-dimensional coatings that require extreme conformality. We describe ALD from the practical and manufacturing viewpoint.