Piezoresistive strain sensors dominate the field of soft elastomer sensors, with many interesting findings and applications. Nevertheless, the methods of characterizing the performance of the sensor differ in each study, leading to different conclusions and making comparison of the different sensor systems challenging. In this Review, the most important methods for characterization of the sensor response are being presented and some cases of elastomer strain sensors are being highlighted. Furthermore, the different materials options for elastomer strain sensors are shown, with special sections for the rapidly growing fields of additive manufacturing and 3D printing. In addition to the material choices and testing methods, different applications of strain sensors are presented. From the biomedical field, these soft sensors find applications in wearable devices, vital sign monitoring, and rehabilitation assistive devices. In soft robotics, they can be used in monitoring and controlling the function of soft robot and actuation systems, a function that can aid with feedback control, increasing the efficiency of the robot’s function. Last but not least, in combination with the emerging self-healing materials, elastomer strain sensors can be used for monitoring the integrity of materials and structures.