Appropriate alignment of the extremity is crucial to maintain efficient use of force and proper load distribution on the joint surface. Disruption of regular alignment causes abnormal pressure zones on the joint surfaces and muscle, ligament and tendon imbalance which triggers the degradation of the cartilage and causing arthrosis in the long term. Sagittal plane deformities are mostly well-compensated by juxta-articular joints. However, efficient movement of the extremity is only possible with correctly aligned anatomic structures. Besides, anatomic alignment of the extremity is also associated with the proprioceptive sensation which is essential to regulate the tension of surrounding muscles, ligaments and tendons. Accurate radiographic measurements should be made to evaluate deformity. Both static and dynamic measurements should be carried out considering the compensation mechanisms. Sagittal plane deformities of the lower extremity directly associated walking and movement and therefore, the ambulatory status of the patient. The objective of this chapter is to explain the diagnosis, management and anatomic and mechanic relations of the sagittal plane deformities.