Electrochemical sensors form a group of very important sensors whose response is a result of an interaction between electricity and chemistry. This chapter provides a detailed description of the three general types of electrochemical sensors: conductimetric, potentiometric, and amperometric. Conductimetric sensors basically consist of an electrically insulating element, such as a sensing platform, upon which one deposits a sensing semiconducting film or metal and metal electrodes. The chapter discusses one of the most important semiconducting metal oxide sensors, the Taguchi sensor, as well as explains the working of chemiresistors and other solid-state electrochemical gas sensors, like the MOS capacitive gas sensor, capacitive polymer gas sensors, and the Schottky diode type gas sensor. It also provides all the necessary tools for the readers to have a firm understanding of electrochemical sensors, thus assisting them in further reading.