In nanoscience, one always has to question how the basic theories we use at the macro- and meso- scales (continuum elasticity and plasticity, etc) apply at the nanometre scale. Studying mechanical properties of nano-size objects also challenges the experimental approaches: how to implement reproducible and controllable mechanical loading? How to measure fields (displacement, stress, strain, etc) with the required nano-resolution? Moreover nanoscience is often tackling the limits of continuum theories and thus simulation is an increasingly important tool to evaluate the different scale transitions needed from atoms to nano-objects. This book provides an updated view of the rapidly growing field of mechanical properties of nano-objects like micropillars, nanowires, nano-particles, nano-precipitates, nano-twins, MEMS, NEMS and structured surfaces, etc. It addresses the theoretical and experimental issues that span the field of mechanics at small dimensions: the fundamentals of continuum mechanics and the mechanics of defects, a general state-of-the-art about modelling and simulation and an overview of the size dependent mechanical properties of nano-objects. Fascinating recent advances are shown in the evaluation of displacement fields at the nano scale by X-ray diffraction and transmission electron microscopy. Examples are given of systems where mastering mechanical properties in small dimensions is key to control the final aimed property (electrical, optical, mechanical, etc).