The development of highly deformable artificial skin sensors is a key technology to advance future robots, such as soft robots, wearable robots, humanoids, and haptic devices. Although conventional force sensors, such as multi-axis load cells, have been widely used in traditional robots with rigid structures, future robots will contain or consist of highly compliant materials in their mechanical structures for human safety, friendliness, and interactions, requiring a new type of sensing schemes for both proprioceptive and exteroceptive feedback. To meet this requirement, we have been investigating various soft skin technologies for sensorizing robots using stretchable materials with embedded sensors. In this paper, we discuss various soft skin sensors we have developed.