The term “modern electrochemistry” has many different definitions after the invention of flexible electrodes. The broadest definition, which seeks to encompass all facets, can be described as science that strives to enhance human existence by utilizing both the bulk of electrolyte solutions and the phenomena that arise at the interfaces between metallic or semiconducting electrodes and electrolyte solutions. Understanding these phenomena can lead to the development and design of new systems or devices that can be used in both healthcare industry and more specialized applications. Examples of these include implanted electricity generators, pacemakers, self-powered electronics, smartphones, and disease detection devices. Faraday’s law, the foundational principle of all electronic sensing devices, begins in this chapter.