Most molecules occurring in living systems (e.g. metabolites, proteins, nucleic acids, lipids and many functionalized carbohydrates) carry discrete fixed charges. Furthermore, resins used for chromatography and biomolecule separation or clay interacting with microorganisms in ecosystems are charged species. Even pure water contains charged species in form of hydroxyl (OH-) and proton (H+) ions. Thus, the specifics of thermodynamic considerations for charged molecules are highly important in life sciences and biotechnology. They are discussed in this chapter. The published material dealing with the thermodynamic properties of strong electrolytes is enormous and would justify a separate book. Therefore, this chapter can only provide an overview and an introduction to some basic principles. The importance of considering electrical charges is illustrated by the following experiment (Fig. 4.1).