Integrating electronics with biological systems presents a technological challenge mainly because the predominant mode of biological signaling relies on ions and molecules of varying size rather than electrons. Iontronics are hybrid devices bridging the gap between electronic and ionic circuits and interactions. These electronic-ionic couplers enable the interaction of electronics with the mainly ion-based biological environment facilitating various applications including smart human-machine interfaces (bioelectronic interface) and energy storage devices. Iontronic devices may not compete with conventional electronic devices in terms of speed due to their typically lower mobility of ions compared to electrons and holes. However, most conventional electronics, particularly digital systems, are usually limited to a binary logic with "on" and "off" states limiting the ability to encode and transmit complex information. In contrast, the charge carriers in iontronics (ionic molecules and species with different valences, sizes, and polarizabilities) can carry specific chemical information and functionality in themselves potentially enabling a selective regulation of various processes in the biological environment in response to specific stimuli (electric/magnetic fields, changes in pH/temperature/viscosity or exposure to light/ultrasound). Therefore, iontronics and iontronic circuits can be beneficial in different applications such as drug delivery (time scale of interest ranging from minutes to hours) , neuromodulation (seconds to hours) or intracellular calcium oscillations (microseconds to days) . Furthermore, ionic computing can mimic biological systems with lower power consumption and increased resistance to external noise together with higher computational accuracy due to the higher mass of ions than electrons (more than 1000 times higher mass). In general, the operating principle and responses of iontronics are based on three types of processes, namely ion diffusion (mostly in nano-confined spaces by constructing asymmetric nanomaterials), electrical double-layer (EDL) formation, and electrochemical reactions. The mentioned operating processes are essential in various iontronic devices and applications ranging from ionic sensors to ionic transistors, and ionic diodes utilizing different electrode materials, such as conductive polymers, piezoelectric materials, nanocomposites, and carbon materials.