This systematic review examines evidence regarding when during the menstrual cycle a woman can initiate combined oral contraceptive (COC) use and what can be done if a woman misses COCs. We searched the MEDLINE and EMBASE databases for articles published from 1966 to March 2005 related to COC initiation and to the effects of late or missed COCs. We identified 11 studies related to COC initiation and 25 studies related to the effects of missed pills. Evidence from these studies suggested that taking hormonally active pills for 7 consecutive days prevents normal ovulation and that initiating COCs through Day 5 of the menstrual cycle suppresses follicular activity. Studies on the effects of missed COCs generally showed that the risk of ovulation is greatest when the pill-free interval lasts >7 days. Limitations of this body of evidence include small sample sizes that may not reflect variation in larger populations, lack of a standard measurement of ovulation and difficulty in discerning how ovulation resulting from late or missed COCs corresponds to the risk of conception.