Untethered soft robots are governed by habitat-specific constraints because operation without physical tethers requires their compliant structures, actuation mechanisms, and energy-delivery pathways to remain compatible with the environment. Adopting a habitat-centric framework, this review compares untethered soft robots across terrestrial, aquatic, aerial, and in vivo environments. On terrestrial substrates, design strategies are shaped by locomotion-specific demands, including contact modulation in crawling, energy storage and release in jumping, switchable adhesion in climbing, and granular fluidization in burrowing. In aquatic habitats, the dominant constraint shifts with depth, from capillary support at the air-water interface, to hydrodynamic drag in shallow water, and to hydrostatic pressure in the deep sea. In aerial environments, strict mass and power limits favor actively flapping insect-scale robots, buoyancy-assisted vehicles, and passively dispersed aeroelastic structures. In vivo, miniaturization and biocompatibility confine the viable design space to magnetic, acoustic, and biohybrid actuation. A multi-environment comparison shows that no single actuation modality is universally optimal. Each habitat filters actuation strategies by the compatibility between energy transmission and the local medium. Material longevity, onboard energy autonomy, multi-environment transition, extraterrestrial operation, and perception and feedback control are identified as cross-cutting challenges whose resolution is required to translate laboratory demonstrations into real-world deployment.
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