This article is primarily concerned with those metallic diffusion coatings that are applied for the protection of iron and steel. It will be presented in three parts: 1. Aluminium coatings. 2. Chromium coatings. 3. Zinc coatings. Special reference is made in each part to the available methods of application, the coatings' composition and structure, their physical properties, and their corrosion and oxidation resistance.