3D Printing and Additive Manufacturing technologies are evolving rapidly. Materials and processes are being created and refined to fabricate compelling and useful objects for a wide array of industries. The digital pipeline from design to fabrication is evolving as well, but there are many challenges associated with matching designers' intent to fabricated objects, especially for advanced visual and mechanical properties. Meeting these challenges will require effort across the entire 3D printing ecosystem and, in some cases, require entirely new ways to specify and encode printing data.