The development of flexible relative humidity (r.h.) sensors attracts considerable interest due to their potential use in wearable electronics for health care applications, such as human respiration monitoring, speech recognition, or measuring epidermal skin humidity. Generally, there are different types of operation principles for humidity detection, such as capacitive-, optical-, quartz crystal microbalances (QCM)-or surface acoustic wave-based sensors, and resistive sensors. Among these, resistive sensors are especially interesting due to their simple readout and easy circuitry integration. Depending on the targeted application, there is, on the one hand, demand for sensors to accurately measure very low humidity levels, on the other hand, also to operate in high humidity environments, like in respiration monitoring. Conventional humidity sensors, which are based, e.g., on conductive polymers, ceramics, or metal oxides, often suffer from non-linearity of their response isotherm, long response-and recovery time, low sensitivity, and large hysteresis, which are crucial parameters for the reliable real-time monitoring of humidity in portable or wearable applications.