Sugar-sweetened soft drinks (SSD) are a special target of many obesity-prevention strategies, yet critical reviews tend to be more cautious regarding the aetiological role of SSD in promoting excess body weight. Since ongoing evaluation of this issue is important, the present systematic review re-examined the evidence from epidemiological studies and interventions, up to July 2008. Database searches of Medline, Cochrane reviews, Google scholar and a hand search of cross-references identified forty-four original studies (twenty-three cross-sectional, seventeen prospective and four intervention) in adults and children, as well as six reviews. These were critically examined for methodology, results and interpretation. Approximately half the cross-sectional and prospective studies found a statistically significant association between SSD consumption and BMI, weight, adiposity or weight gain in at least one subgroup. The totality of evidence is dominated by American studies where SSD consumption tends to be higher and formulations different. Most studies suggest that the effect of SSD is small except in susceptible individuals or at high levels of intake. Methodological weaknesses mean that many studies cannot detect whether soft drinks or other aspects of diet and lifestyle have contributed to excess body weight. Progress in reaching a definitive conclusion on the role of SSD in obesity is hampered by the paucity of good-quality interventions which reliably monitor diet and lifestyle and adequately report effect sizes. Of the three long-term (>6 months) interventions, one reported a decrease in obesity prevalence but no change in mean BMI and two found a significant impact only among children already overweight at baseline. Of the six reviews, two concluded that the evidence was strong, one that an association was probable, while three described it as inconclusive, equivocal or near zero. Reasons for some discrepancies are presented.