Nowadays, coplanar waveguide-to-stripline transitions in PCB technology can be easily designed from DC up to 30 GHz. However, emerging microwave and mm-wave circuits operating at frequencies above 30 GHz will require conveniently designed transitions, able to operate up to 67 GHz while entirely compatible with standard PCB manufacturing technologies. The main goal of this MSc thesis is to analyse coplanar waveguide-to-stripline transitions for ultra-wideband operation (i.e., from DC up to 67 GHz) using multi-layer PCBs. The main challenge of this work is to envisage novel structures and to develop new insight into the materials, stack-up, geometry, and fabrication issues to be considered for improving the electrical performance.