Ionic polymer metal composites (IPMCs) are a class of smart material that can be used as both actuators and sensors. In this chapter, we explain the basic mechanisms of the self-sensing ability of IPMCs. Generally, three main properties of IPMCs are used for sensing: mechanical-to-electrical transduction, surface resistance and the resistance across the IPMC. For the first technique, mechanical stimuli generate a detectable voltage that can be used as sensing signal. The second technique exploits the asymmetric change in the surface resistance of each side of the IPMC during bending. The last technique is based on the fact that IPMCs behave as having simple resistance for high-frequency voltages. The first two techniques are studied widely, and here we give a short review of each, but the last technique is introduced for the first time. In this chapter, we also highlight the advantages and disadvantages of each technique.