Treatment-related multiple pregnancy poses the biggest threat to the safety of IVF. Despite a double embryo transfer (DET) policy in most European centres, twin rates continue to be unacceptably high, at 20-35%. Elective single embryo transfer (SET) is an effective way to minimize twin pregnancies, but the debate surrounding its routine clinical use continues. A review of the literature was undertaken in order to seek evidence about the effectiveness of SET, and identify barriers to its acceptance in clinical practice. Data from randomized controlled trials (RCTs) indicate that SET results in lower live birth rates per fresh IVF cycle (odds ratio 0.53; 95% confidence interval 0.31-0.89; P = 0.02) in comparison with DET. Data on cumulative live birth rates are unavailable from RCTs, although the expectation is that these are comparable in the two groups. SET is unlikely to be suitable for all women undergoing IVF and outcomes may be sensitive to different laboratory protocols. The perceived effectiveness of SET is influenced by the way existing evidence is interpreted. Other factors affecting the routine use of SET include laboratory techniques, individual preferences and funding issues.