This chapter explains the possible causes of damage to the cauda equina or conus medullaris causing cauda equina syndrome (CES). Spinal fracture was the main etiology in young men, disc herniations in middle-aged patients and iatrogenic lesions (i.e. spinal stenosis surgery) in older patients. Animal studies have demonstrated a greater resistance to acute pressure damage of small- compared to large-diameter nerve fibers and equal susceptibility of large-diameter motor and sensory fibers. This phenomenon goes some way to explaining patients with sensory loss but preserved pelvic organ function but not vice versa. Parasympathetic fibers within cauda equina are even thinner, and therefore might be even more resistant to compression than sensory fibers. At acute presentation careful neurologic examination is needed, with emphasis on the lower sacral segments and urgent MRI of the lumbar spine. Insertion of a urinary catheter is needed to check for urinary retention and to drain the bladder.