Most of today's robots have rigid structures and actuators which require complex software control algorithms and sophisticated sensor systems in order to behave adaptable, compliant, and safe in contact with unknown environments or with humans. Moreover, in terms of energy efficiency, peak force and speed, these robots are still considerably weaker than their biological archetypes. An alternative design approach is to build actuators with physically adjustable compliance and damping, which are able to store and release mechanical energy, react softly when touching the environment, and provide an intrinsic degree of safety, just like muscles do. The ability to vary the impedance (i.e., stiffness and damping) is crucial in order to optimally adapt to a large variety of situations. However, the technological realisation of such systems is very challenging, having many open problems in terms of materials, design, actuation, and control concepts. VIACTORS addresses the development and use of safe, energy-efficient, and highly dynamic variable-impedance actuation systems which will permit the embodiment of natural characteristics, found in biological systems, into a new generation of mechatronic systems. Target outcome of the project is that of obtaining the intended physical interaction and motion behaviours of the robotic system intrinsically by its physical structures to the maximum extent possible. This will not only save computational and communication costs for controlling the robot motion; it will also allow us to match the task requirements in a natural and highly dynamic way as it can be observed in biology. And, perhaps most important, to provide a powerful, human-like physical interface which can be accessed by higher (cognitive) intelligence levels without having to care for basic motion generation principles. This advance in technology will pave the way towards new application fields, such as industrial co-workers, household robots, advanced prostheses and rehabilitation devices, and autonomous robots for exploration of space and hostile environments. Therefore, results of this project will deeply impact applications where successful task completion requires people and robots to collaborate directly in a shared workspace or robots to move autonomously and as efficiently as humans.