The pelvis is a flexible, dynamic stabilizer between the lower lumbar spine and the hip joints. Depending on the body posture or specific disease or deformity affecting the spine or lower limbs, the pelvis will compensate in the coronal and sagittal planes to maintain balance and minimize energy expenditure. If such rotational adaptations of the pelvis are not considered during total hip arthroplasty, the result may be malalignment of the acetabular component, functional instability, and a perceived limb-length discrepancy postoperatively. Moreover, improper orientation of the acetabular cup can have even more severe consequences if metal-on-metal bearing surfaces are used due to the potential for metallic wear debris. The final position of the acetabular cup in total hip arthroplasty must not only be appropriately oriented in three dimensions, but also dynamic and functional for optimal outcome. This chapter discusses the dynamic relationship between the pelvis and the spine and its application in total hip joint replacement.