The primary biomechanical function of the lumbar spine is to bear the weight of the torso, head-neck, and upper extremities and support physiologic movement. The lumbar spinal column resides vertically between the thoracic spine and sacrum, and consists of five bony vertebrae interconnected by soft tissues including the intervertebral discs, ligaments, and muscles to maintain the integrity of the column under physiologic and traumatic environments. Injuries secondary to excessive deformations or loading resulting from external dynamic forces such as falls, or in military environments, aviator ejections, helicopter crashes or underbody blasts, can result in fracture of the lumbar spine with or without mechanical and clinical instability, and loss of normal function. These types of injuries can have significant consequences for the patient. Mechanically-induced traumas are transmitted to the lumbar spine in a variety of different ways. For example, axial or eccentric compressive forces transmitted to the lumbar spine through a vehicle seat sustaining high-rate vertical acceleration may result in different fracture types (e.g., burst fracture versus anteriorly-oriented wedge fracture), lead to mechanical instability, and impair normal daily activities. These acute consequences are in addition to the chronic effects of lumbar spine trauma including chronic back and lower extremity pain due to spinal degeneration, spinal cord or nerve root injury, or loss of lower limb sensation and function. This chapter outlines lumbar spine injury classification including mechanisms and clinical implication, describes experimental techniques used to understand injury mechanics, and provides a listing of biomechanical fracture tolerance and injury criteria from experimental studies incorporating human cadavers. Due to the breadth of literature on lumbar spine injury mechanics, this chapter is not intended to be comprehensive. Rather, the reader will be provided with a overview of concepts relevant to the contemporary understanding of lumbar spine injury mechanics and tolerance.