**1. Introduction**

In this chapter, the fundamentals of nanoionic redox based resistive switching materials are described including different modes of switching. The primary parameters have also been discussed which essentially affect the resistive switching behavior. In addition, this chapter encompasses the various physical as well as chemical phenomena occurring at nano level during resistive switching of devices and other related trending technological areas are also included. An introduction to three distinct kinds of redox based resistive switching materials is given followed by their short history and promising applications into device fabrication field. Further, a brief discussion related to requirement and optimization of device performance parameters has been incorporated along with future prospects, challenges and important industrial applications such as memory, logic circuits and so on. The elements that observe resistive switching are driven by a reversible phenomenon taking place between two terminals. This behavior depicts primarily two different resistance values of nonvolatile nature depending on the external electrical bias conditions [1]. However, such a reversible behavior obtained under continuous applied external stimuli can also be achieved within more than two resistance levels commonly called as multilevel resistive switching. The term 'nonvolatile' suggests the retention or preservation of change in resistance level after removal of the external stimuli. Such memristive materials are capable of memorizing these resistance values under the influence of stimuli [2]. Similar to phenomenon of resistive switching, there can be different

possible switching scenarios such as magnetoresistive phenomenon i.e. spin-transfer torque, electrical effects like the leakage current *via* gate oxide layer containing trapped defects, change in structure/phase among amorphous and crystalline phases, and nanoionic redox phenomenon [1]. So, this chapter deals with the phenomenon of nanoionic redox realized due to ion movement within the two terminal based device structure elements and results into different resistance values of the material. In this chapter, a few terminology such as redox-based resistive switching random access memory (RRAM), resistive switching and/or memristive etc. is to be used frequently. Such terms have been essentially included to explain the fundamental physical characteristics of materials showing potential in multilevel switching or analog properties.
