Carrying a load increases the metabolic costs of movement: exoskeletons are devices developed primarily to minimize the operatorpsilas energetic contribution for the action. The rise of the exoskeletons research can be set in the second half of the past century. The most important contribution given by these early attempts to develop supporting structures for human use, was to identify and focus the researcherpsilas attention on some key aspects, fundamental for a further implementation of these devices. Among them, the importance of efficient and small supply systems, of precision and control, o simple human interface and of the knowledge of locomotion biomechanics. The aim is a technological approach capable to match the artificial system with the human beings with minimum interactions and maximum synergy. This work deal with the state of art of the projects facing with the development of exoskeletons and active orthoses for the lower limbs. It focuses the actuators problem showing advantages and disadvantages of a variety of technological alternatives. The work is the starting point for a new exoskeleton design.