This thesis presents the design for a novel compact crab cavity for the HL-LHC upgrade at CERN, Geneva. The LHC requires 400MHz RF cavities that can provide up to 10MV transverse gradient across two to three cavities with suitably low surface fields for continual operation. As a result, a cavity design was required that would be optimised to these new parameters. From initial design studies based on Jefferson Laboratory’s CEBAF deflector, extensive optimization was carried out to design a superconducting crab cavity, dubbed the Four Rod Crab Cavity (4RCC). The design underwent several iterations throughout the course of the project due to changing requirements from CERN, particularly space requirements inside the LHC. In addition, it was decided that a focus on field flatness was required. An aluminium prototype was then constructed from the finalised and computer-simulated design to confirm the designed field flatness. Additional computer simulation studies using CST were performed to ensure that multipacting and higher order modes were at tolerable levels. Design considerations were made to ensure a niobium prototype could be constructed for cold testing, the results of which are presented along with discussion of future plans for continuing to further the design of the cavity.