The current prostheses for primary total knee arthroplasty (TKA) have all come from three main designs: PCL sparing, posterior stabilized, and mobile bearing. Since the very first TKA by Insall, Ranawat, and Walker, the issue of the cruciate ligaments and their importance in knee range of motion (ROM), balance, and proprioception has been ever present. The original total condylar knee sacrificed both cruciates and relied upon the surface anatomy for stability. The polycentric knee attempted to retain both cruciates and was fraught with loosening. Then, designers decided to sacrifice the anterior cruciate ligament and rely upon the posterior cruciate ligament (PCL) for stability or resect both cruciates and substitute for the PCL. The mobile bearing design looked toward addressing wear with mobile surfaces and initially sacrificed the cruciate ligaments. The 15 percent dissatisfaction rate continues to trouble designers and the final solution is yet to come. This chapter addresses the present-day prostheses including PCL sparing, medial pivot, PCL substituting, bicruciate retaining, partial knee replacements, and constrained knees.