The importance of coordinated midlines often is unappreciated as it relates to the treatment plan for the orthodontic case. All three of the patient's midlines--facial, maxillary, and mandibular--must be considered if ideal correction is to be achieved. Proper differential diagnosis of the cause will allow the practitioner to appropriately use either inter- or intraarch mechanics for the resolution of midline discrepancies. Midline correction should be undertaken from the initiation of treatment and once all midlines are coordinated they should be maintained as a guide for any further force systems used in completing the case. Functional, dental, and iatrogenic midline discrepancies are discussed pertaining to their diagnosis and treatment.