Electrochemical sensing devices are composed of various components. Three-electrode systems used for analyzing electrode reactions are fabricated via thin-film and thick-film techniques. Solid-state ion-selective electrodes (ISEs) that do not contain any internal electrolyte solution facilitate miniaturization. In such ISEs, ion-to-electron transduction at the ion-selective membrane/electric conductor interface is required. For this purpose, materials such as conducting polymers and carbon nanomaterials were used. Ion-sensitive field-effect transistors with impedance transformation function were also used. Reference electrodes are essential for setting the potential in the three-electrode systems and measuring the potential of the ISEs. The potential stability and reproducibility requirements are particularly stringent while performing potentiometric sensing using the ISEs. To fabricate such miniature reference electrodes, numerous microfabricated Ag/AgCl electrodes have been reported. To integrate such electrochemical components in microfluidic systems, electrical connection of two different solutions often poses a problem. Although a liquid junction prepared with a hydrogel can be used, there are associated problems like mixing of components on both sides and generation of liquid-junction potentials. To address these issues, a metal connection in which the potential difference between the end and the solution on both sides can be determined by redox reactions is effective. Electrochemical oxygen and carbon dioxide sensors employing gas-permeable membranes can also be fabricated. Depending on gases, appropriate gas-permeable membrane materials with good gas permeability and suitability for microfabrication may not be found. In that case, air-gap structures formed using microfluidic structures can be used as an alternative to the gas-permeable membrane in order to achieve fast responses.