**1. Introduction**

Electrochemical impedance spectroscopy (EIS) is a usually described as a *potent* (if not the most powerful) electrochemical analytical technique. The history of EIS goes back to the late nineteenth century, thanks to the foundations established by Heaviside on his work on the linear systems theory (LST). By the end of the same century, the success achieved by Warburg to broaden the conception of *impedance* to the electrochemical systems (ES) came to the scene. It was close to the middle of the twentieth century, when the EIS started to realize its potential! That came with the invention of the potentiostat in the 1940s, followed by the frequency response analyzers in the 1970s. This progress has led to the application of EIS chiefly in investigation of corrosion mechanisms [1–3].

Later on, this has opened the doors for realms of applications of EIS. Applications encompassed electrocatalysis and energy [3–5]; characterization of materials, e.g. corrosion phenomenon surveillance [6, 7]; and depiction of quality of coatings [8], exploring mechanisms of processes such as electrodeposition and electro-dissolution [9, 10], food and drug analysis [11–13], detection of biomarkers [14, 15], and water analysis [16, 17].

It is noteworthy to mention that impedance spectroscopy (IS), depending on the material used, the device, and the system or process to be studied, has two main categories: EIS (the topic of this chapter) and dielectric IS. A major difference is that EIS applies to systems/materials involving chiefly ionic conduction, in contrast to electronic conduction in the case of dielectric IS. Therefore, it can be observed from the fields of EIS applications that EIS usually applies to systems like electrolytes (solid/liquid), polymers, and glasses [18–21].

In general, EIS measurements involve the application of an alternating current (AC) voltage or current to the system under investigation, followed by measurement of the response in the form of AC current (or voltage) as a function of frequency. Measurements are usually performed using the potentiostat, and the measured response is analyzed using a frequency response analyzer (FRA) [18]. By and large, three factors make EIS exceptionally attractive in terms of applications:


A combination of the three advantages led to the wide use of EIS as previously mentioned.

The current chapter throughout the following sections is investigating the applications of EIS in a variety of matrices, mainly in food, drug, and water analysis, and the recent advances in these fields as well as comparisons between EIS and other electroanalytical approaches applied for the same purposes.
