Metamaterials are materials especially engineered to have a peculiar physical behaviour, to be exploited for some well-specified technological application. In this context we focus on the conception of general microstructured continua, with particular attention to piezoelectromechanical structures, having a strong coupling between macroscopic motion and some internal degrees of freedom, which may be electric or, more generally, related to some micro-motion. An interesting class of problems in this context regards the design of wave-guides aimed to control wave propagation.The description of the state of the art is followed by some hints addressed to describe some possible research developments and in particular to design optimal design techniques for bone reconstruction or systems which may block wave propagation in some frequency ranges, in both linear and non-linear fields.Metamaterials are materials which are designed to have exotic behaviour: the concept has been first conceived for optical devices. Therefore, very often one talks about mechanical metamaterials, when the exotic behaviour is limited to mechanical effects, as e.g. very negative Poisson effects. This paper is based on a really simple idea: construct a bridge between two different cultural environments which address the same relevant problems. Metamaterials are studied and conceived by physicists to tackle problems and applications not yet considered in engineering sciences; at the same time the community of continuum mechanics nearly completely ignores what physicists devise and develop in the same field. This review intends to fill a gap in order to stimulate a parallel development of the these theories and to near not communicating scientific groups.The mathematical formalism chosen thereinafter is that preferred by physicist, like Landau type variational principles, and the treated subjects are chosen, in the opinion of the authors, from those considered nowadays more important by applied mechanicians.The capability of continuum theories to describe the time evolution and the deformation of the micro-structure of complex mechanical systems was recognised in the very first formulations of continuum mechanics, as in the pioneering work by Piola (Piola, 1846). He was lead by stringent physical considerations to introduce higher gradients of displacement field, as necessary independent variables, in the constitutive equation for the deformation energy of continuous media. For a more modern interpretation of this subject refer to (Mindlin, However in a similar period, while Piola was producing his papers, Cauchy and Poisson obtained a description, with a very elegant and effective format, for continuum mechanics in which: i) the displacement from a reference configuration is the only kinematic descriptor;ii) the crucial conceptual tool is Cauchy stress which is constitutively related only to the first gradient of displacement;iii) the crucial postulates are those concerning balance of mass, linear and angular momentum and, when necessary, energy.T...