Piezoelectric ceramics are becoming used increasingly as the basis for electromechanical sensors and actuators for control, medical, electronic and microelectronic machine (MEMS) applications. Electromechanical actuators take benefit from the strain resulting from the application of an electric field in ferroelectric materials. Many sources of internal stress can arise in actuation devices. First the manufacturing process can introduce residual stresses. The boundary conditions related to the actuator packaging is another source of operating stress. At a finer scale still, due to the heterogeneity of ferroelectric materials (polycrystalline structure), the piezoelectric strain is usually not compatible, resulting in internal stresses when an electric field is applied. Despite its significant role, the dependence of the internal stress on the piezoelectric strain is rarely accounted for in the design of actuators, mainly because it is difficult to quantify or predict. The development of micro-macro models of ferroelectric behaviour provides a pathway to establish fully coupled electro-mechanical constitutive laws for ferroelectric materials. Such constitutive laws will improve the quantitative description of electric field induced strains, and allow the optimisation of piezoelectric actuator design. Consequently, through the development of multiscale tools the objective of this project is to describe in a quantitative way the effect of applied and internal stress on ferroelectric behaviour. This will provide the tools to design high performance ferroelectric actuators.