Continuous labor support offers multiple benefits for mothers and infants. The type of caregiver that is the best support person in labor has not been identified. A critical review of the English language literature was conducted to describe the current state of knowledge on different types of labor support persons. Randomized trials and other published reports were identified from relevant databases and hand searches. Studies were reviewed and assessed by using a structured format. Eight randomized trials met the selection criteria for inclusion in this analysis. These trials investigated untrained and trained lay women, female relatives, nurses, lay midwives, and student lay midwives as labor support persons. Support by untrained lay women starting in early labor and continuing into the postpartum period demonstrates the most consistent beneficial effect on childbirth outcomes. However, more randomized controlled trials are warranted before firm conclusions may be drawn.