This chapter highlights the development of theories for interpreting electrochemical measurements which span over one hundred years. It began when Volta and Nicholson constructed batteries and Davy used batteries to reduce metals and generate gases. Davy developed an electrochemical series making use of the periodic table of elements. Faraday gave further structure to the observations. Thompson showed electricity is made of electrons. Nernst demonstrated a relationship between electrolyte concentrations and cell potentials and Gibbs developed the formulas of thermodynamics and explained electrical work in terms of charge passed from one potential to another. Tafel, Butler, and Volmer adopted current flow measurements to analytic formulas. Schrödinger’s wave equation provided the understanding of electron behavior which was used by Gurney to explain electron transferElectron transfer at electrode surfaces. The analytic formulas cell potentials from concentrations and for current flow from potentials remain foundational to electrochemical science and engineering.