3D printing technologies are currently changing manufacturing, introducing novel design and fabrication rules for building complex and highly interconnected devices. These technologies rely on the fabrication of 3D objects in a layer-by-layer approach starting from a digital model. 3D printing technologies are rapidly evolving from prototyping tools to large-scale manufacturing technologies, also in photonics and optoelectronics. In these fields, main challenges are related to the specific requirements in terms of (a) optical properties of the used materials, either passive or active, (b) spatial resolution needed for achieving smooth surfaces and homogeneous 3D structures, and (c) uniformity of interfaces between diverse materials. Moreover, most of the optical components currently available by 3D printing are passive and static, i.e. they are designed to perform a specific task, and cannot adapt themselves to the varying conditions of their surrounding environment. xPRINT aims at introducing novel additive manufacturing approaches for printing 4-dimensional optical components, namely devices which have complex 3D architecture and optical properties that can change in time in response to external stimuli. xPRINT will achieve such objective through a synergistic approach, encompassing modelling and diagnostics of the 3D printing processes, as well as advanced process engineering, specifically targeting photo-active materials.