This chapter reviews the methods and applications of diffusion coatings. Diffusion coatings can offer a low-cost solution to providing environmental protection or can form a part manufacture in more complex, more sophisticated coating systems. First, the chapter reviews the pack cementation process, a traditional route to apply diffusion coatings with standardized process established for aluminizing, chromizing, and depositing zinc-based coatings (sherardizing). Using the pack aluminizing process as an example, coating microstructures are discussed in terms of process parameters that control the chemical vapor deposition process. Alternative process routes are reviewed, including slurry cementation, fluidized-bed techniques, metalliding, the use of molten salt baths, and gas and vapor phase processes. With this process understanding, coating–substrate interdependency, the ability to codeposit materials, and the most recent developments aimed at improving the high temperature stability and durability of aluminide are explored; particularly, the formation of platinum aluminides and silicon aluminides and the most recent work on adding reactive elements into the aluminizing process. Such developments have seen the use of diffusion coatings in part manufacture of more complex coating systems, including bondcoats under thermal barrier coatings for high-performance gas turbines through to oxidation and corrosion protections for industrial gas turbines, medium speed diesels, automotive applications, and chemical and petrochemical plant.