Abstract: Flexible sensors have potential applications in the fields of wearable devices, implantable devices, and electronic skin, etc. Polymer hydrogels have biomimetic mechanical properties and biocompatibility, and thus are widely recognized as promising candidate materials for biomimetic flexible devices. Conductive hydrogels, including electron conductive and ion conductive hydrogels, have been widely developed as fundamental materials for flexible sensors. The mechanical properties and sensory performance can be conveniently manipulated by designing the network structures of hydrogels to meet the demands of diverse applications. Linear sensitivity, wide working range, linearity, low limit of detection and stability are pursued by designing not only the network structure, but also the device configurations and microstructures. Moreover, the device-tissue interface is critical for implantable applications for monitoring the motions and health of tissues and organs. This review systematically accounts the latest progress in the field of hydrogel sensors including our studies on tough conductive hydrogel sensors, paying special attention to structure-performance relationship, hydrogel-tissue adhesion, and application for human organ motion monitoring. Interpenetrating conductive/hydrophilic networks provide not only high strength and toughness, but also low percolation conductivity, high conductivity and linear sensitivity over a broad working range. Ion conductive hydrogels based on zwitterionic polymers show very high stretchability and ion conductivity, self-healing, and sensitivity, primarily attributed to the dipole-dipole interaction and ion conduction channels. Such zwitterionic sensors are adhesive to biotissues, and enable real-time monitoring of organ motions. This review aims to inspire new ideas to develop novel high performance hydrogel sensors, shed new insights into mechanisms behind the sensory performances, and boost hydrogel devices toward practical applications. Major challenges and future opportunities in this rapidly developing field are outlooked.