The ability to three-dimensionally pattern semiconducting electronic and optoelectronic materials could provide a transformative approach to creating active electronic devices without the need for a cleanroom or conventional microfabrication facilities. This could enable the generation of active electronics on-the-fly, using only source inks and a portable 3D printer to realize electronics anywhere, anytime, including directly on the body. Indeed, interfacing active devices with biology in 3D could impact a variety of fields, including biomedical devices, wearable electronics, bioelectronics, smart prosthetics, and human-machine interfaces. Developing the ability to 3D print various classes of materials possessing distinct properties will enable the freeform generation of active electronics in unique functional, interwoven architectures. Yet, achieving seamless integration of these diverse materials via 3D printing is a significant challenge which requires overcoming discrepancies in material properties in addition to ensuring that all the materials are compatible with the 3D printing process. We will present a strategy for three-dimensionally integrating diverse classes of materials using a custom-built 3D printer to create fully 3D printed device components built around active electronics. As proof of concept, we have 3D printed quantum dot-based light-emitting diodes (QD-LEDs), polymer-based photodiodes on curvilinear surfaces, flexible displays, and skin-interfaced hybrid devices. These results represent a series of critical steps toward the 3D printing of high performance, active electronic materials and devices.