The concept of 3D chip stacks has been advocated by both industry and academia for many years, and hailed as one of the most promising approaches to meet ever-increasing demands for performance, functionality and power consumption going forward. However, a multitude of challenges has thus far obstructed large-scale transition from "classical" 2D chips to stacked 3D chips. We survey major design challenges for 3D chip stacks with particular focus on their implications for physical design. We also derive requirements for advances in design automation, such as the need for a unified workflow. Finally, we outline current promising solutions as well as areas needing further research and development. bly design kit; design standards 2. 3D-CHIP STACKING OPTIONS AND HIGH-LEVEL DESIGN CHALLENGES 3D-chip stacking falls into four categories (Fig. 2): (i) package stacking, (ii) interposer stacks, (iii) through-silicon via (TSV)based 3D ICs, and (iv) monolithic 3D ICs. Each option has its This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike International 4.0 License.