Low back pain is a frequent complaint of patients seeking care at private offices, ambulatory clinics, and EDs. Key signs and symptoms can guide clinicians in differentiating acute and persistent mechanical low back pain from back pain resulting from a specific cause. Awareness of these findings can reduce the number of missed or incorrect diagnoses that lead to poor patient outcomes. The ability to recognize the red flag findings of serious causes of low back pain ensures prompt diagnosis and initiation of appropriate treatment. This article highlights the history and physical examination findings that will improve identification of red flags associated with emergency or serious causes.