Scientific inquiry is a never-ending process, and, as a result, the investigation of a particular chemical concept is rarely complete. As new technology is invented and, subsequently, applied in research laboratories, chemists are able to re-evaluate pre-existing data, in order to refine established definitions, identify new trends and patterns, and develop a deeper understanding of the molecular-level interactions that occur during chemical and physical processes. The experimental evidence that is collected during a scientific study often supports previously-developed theories. However, an investigation may yield one or more unexpected results that expose limitations in or, in extreme instances, disprove accepted chemical principles. As a result, scientists must establish new theories and definitions that reflect an updated understanding of their newly-collected experimental data. As will be discussed in greater detail in the following three sections of this chapter, the initially-established characterizations of acids and bases were revised, twice, in order to eliminate experimental and conceptual limitations that are inherently-present in their definitions. Each acid/base classification, which is named after the chemist, or chemists, responsible for its development, is utilized in distinctive applications, which will be discussed in greater detail in the remaining sections of this chapter.