Chapter 5 deals with the subject of water quality. First of all, the importance of water quality for commercial dishwashing is discussed. Among other things, it describes the different constituents of the water and their influence on the dishwashing result. Furthermore, recommended values/limit values for individual ingredients are listed, which have proven themselves in practice for optimal dishwashing results. Furthermore, different water treatment options, such as water filtration, water softening, partial water desalination and full water desalination are listed and their functions are explained in detail.