Abstract
This chapter provides some examples that illustrate the potential of the concept of four-dimensional printing. The examples are chosen to illustrate the generalized definition of the fourth dimension within the technology. The exposition starts by introducing the general concepts that evolved out of all printing polymeric materials. Therefore, the authors provide illustrations of the concept of material morphing which stands at the core of the conventional definition of 4-D printing. Following such an illustration they discuss another emerging definition that casts 4-Dimensional printing in terms of layer-by-layer functional gradients. Such a definition, which is the future of this particular aspect of AM, is well suited for the printing of metallic objects. It is based on functional customization of material properties within each printer layer to ensure a pre-planned behavior of the printed structure upon the application within its intended environment.