The lumbar spine differs from other spinal regions due to its distinct biomechanical features. These include large, weight-bearing vertebral bodies and facet joints oriented sagittally, which allow for high axial loading and multidirectional movement. The interactions between anatomical structures, physical principles, and clinical implications related to lumbar spine biomechanics are examined in this chapter. The instantaneous axis of rotation, the functional spinal unit, and the function of intervertebral discs in load distribution are key concepts that will be explored. Spinal instability, a critical concern in lumbar spine disease, manifests differently across conditions such as disc degeneration, stenosis, and trauma. Ultimately seeking to optimize treatment strategies and improve patient outcomes in lumbar spine management, this chapter bridges theoretical biomechanics with clinical application, offering insights into how these principles inform surgical decision-making, implant design, and postoperative care.