The purpose of this project is the realisation of a prosthetic foot, able to adapt to the different conditions encountered by the patient during walking. The analysis of the human gait shows that a normal foot has the capability to assume an optimised shape. Coordinated movements of body parts, together with the shape assumed by the foot, result in minimising the energy expenditure during walking. Concerning the prosthetic foot, the bending stiffness of the plantar spring is the quantity to vary, in order to achieve the desired change in foot properties. Three different concepts are investigated: The first one deals with the possibility of adjusting the static response of the plantar spring of the foot by means of solid-state actuators. Piezoceramic patches are employed as active elements which are able to change the shape of the plantar spring. The second concept takes advantage of the property of a hollow beam with an ellipticalal cross- section. Depending on the value of the inner pressure applied, the ellipticalal cross-section deforms, resulting in a change in the moment of inertia of the cross-section itself. The deformation of the cross-section, due to the inner pressure, leads to an increased bending stiffness of the structure. The behaviour of an integrated structure made of a passive plate and of active beams is numerically and experimentally analysed. The third concept is based on the possibility of controlling the deformation of a flat hollow structure while under a bending load. The conventional plantar spring is replaced by a thin structure consisting of two parallel plates. The space between the plates is filled with a hydraulic fluid to generate inner pressure. Due to the shear stiffness of the structure, the bending load leads to the inward deformation of the plate assuming the shorter radius of curvature. The inner pressure stiffens the structure by controlling the inward displacement of the plate. A system for generating the pressure is presented. The system is activated by the patient and takes advantage of the work performed during the stance phase. The system is integrated in the foot structure and may replace the conventional ankle joint of the prosthetic foot.