Effective locomotion and maneuvering in aquatic environments is important for survival for marine fauna. The ability to move quickly, change direction, and tune energy consumption for long migrations is critical for escape from predators and pursuit of prey. This controlled propulsion in terms of varying speed, turning rates, and actuation effort is of interest for the next generation of underwater vehicle design. Integration of biological functional simplicity, robustness, and superior performance enables robotic vehicles to successfully complete difficult and dynamic operational goals. Gelatinous animals known as Cnidarians employ a wide variety of propulsive methods, ranging from the simple but efficient propulsion of large jellyfish to the rapid and highly maneuverable multi-jet propulsion of colonial animals known as siphonophores. This dissertation studies how these two extremes of underwater soft body propulsion are able to achieve simple yet effective control and locomotion, and thus inform the design of effective vehicle propulsion control and actuation. Two large single bell jellyfish robots, Cyro 2 and Cyro 3, were designed and constructed to implement the simple body form and propulsive methods of large jellyfish to study the unique locomotive characteristics and fluid interactions that generate straight swimming and turning maneuvers. The other extreme of small soft-body colonies moving by multi-jet propulsion was subsequently investigated in-depth, starting with a characterization of the biological fluid jetting actions and gaits. The results of these performance capabilities were then applied to an experimental robotic model with bio-inspired construction and controls to verify an elegant but highly functional neurological control scheme and the kinematic capabilities from varying jetting gait patterns. / PHD / The ability to move rapidly in any direction is a primary characteristic of successful animal species. Evasion of predators, as well as pursuit of prey, is paramount for survival. Jellyfish are excellent examples of animals that have thrived for millions of years with varied methods of moving in their diverse environments. However, the propulsion methods of large jellyfish in straight swimming and turning have not been well understood until recent years. This dissertation focuses on the fundamental understanding of the locomotion and fluid interaction that jellyfish use for propulsion. A large jellyfish robot, named Cyro 2 (“Cy” for the species Cyanea, “ro” for robot, and the second generation of the design), was constructed to explore the role of various structural and fluidic parameters on the locomotion characteristics of the largest jellyfish species Cyanea. The successor Cyro 3 was designed to mimic the complex motions of large jellyfish during maneuvering. Motion tracking and fluid analysis of the robot during turning was utilized to explain how jellyfish dynamically control their orientation. These results inspired further study of a unique relative of jellyfish, the siphonophore, which can swim with a modular chain of soft pumping bodies that coordinate without a central nervous system. This unique control strategy and method of movement underwater was studied by analyzing specimens of the siphonophore Nanomia. Development and modeling of elegant control techniques inspired by this species is presented and implemented on an experimental model that uses this unique propulsion method to validate and expand upon observations of live specimens. Combined, the results obtained in this dissertation open the possibility of designing advanced underwater vehicles.