Although the benefits of breastfeeding are widely accepted, the effectiveness of different strategies to promote the continuation of breastfeeding once initiated are less clear. The objective of this systematic review was to describe studies comparing standard care with the provision of extra breastfeeding support and to measure its effectiveness. Outcome measures used were rates of cessation of any breastfeeding or exclusive breastfeeding at chosen points in time. Measures of child morbidity and maternal satisfaction were also used when these were reported. Twenty eligible randomised or quasi-randomised controlled trials were identified, involving 23 712 mother-infant pairs. Extra support had a beneficial effect on the duration of any breastfeeding (RR [95% confidence intervals] for stopping any breastfeeding before the last study assessment up to 6 months 0.88 [0.81, 0.95]; 15 trials, 21 910 women). The effect was greater for exclusive breastfeeding (RR for stopping exclusive breastfeeding before the last study assessment 0.78 [0.69, 0.89]; 11 trials, 20 788 women). Although the point estimates of relative risk were very similar, benefit derived from professional support achieved statistical significance for any breastfeeding (RR 0.89 [0.81, 0.97]; 10 trials, 19 696 women) but not for exclusive breastfeeding (RR 0.90 [0.81, 1.01]; six trials, 18 258 women). Lay support was effective in reducing the cessation of exclusive breastfeeding (RR 0.66 [0.49, 0.89]; five trials, 2530 women) while the strength of its effect on any breastfeeding was less clear (RR 0.84 [0.69, 1.02]; five trials, 2224 women). Professional support in the largest trial to assess health outcomes produced a significant reduction in the risk of gastrointestinal infections and atopic eczema. In two trials with children suffering from diarrhoeal illness, extra support was highly effective in increasing short-term exclusive breastfeeding rates and reducing recurrence of diarrhoea. This review supports the conclusion that supplementary breastfeeding support should be provided as part of routine health service provision. There is clear evidence for the effectiveness of professional support on the duration of any breastfeeding although the strength of its effect on the rate of exclusive breastfeeding is uncertain. Lay support is effective in promoting exclusive breastfeeding although the strength of its effect on the duration of any breastfeeding is uncertain. Evidence supports the promotion of exclusive breastfeeding as central to the management of diarrhoeal illness in partially breast-fed infants.