A systematic review of randomized control trials (RCT) was undertaken to evaluate the effectiveness of eHealth behavioral interventions aiming to improve smoking rates, nutrition behaviors, alcohol intake, physical activity levels and/or obesity (SNAPO) in young adults. Seven electronic databases were searched for RCTs published in English from 2000 to April 2015 and evaluating eHealth interventions aiming to change one or multiple SNAPO outcomes, and including young adult (18-35years) participants. Of 2,159 articles identified, 45 studies met the inclusion criteria. Most interventions targeted alcohol (n=26), followed by smoking (n=7), physical activity (n=4), obesity (n=4) and nutrition (n=1). Three interventions targeted multiple behaviors. The eHealth interventions were most often delivered via websites (79.5%). Most studies (n=32) compared eHealth interventions to a control group (e.g. waiting list control, minimal intervention), with the majority (n=23) showing a positive effect on a SNAPO outcome at follow-up. Meta-analysis demonstrated a significantly lower mean number of drinks consumed/week in brief web or computer-based interventions compared to controls (Mean Difference -2.43 [-3.54, -1.32], P<0.0001, n=10). Sixteen studies compared eHealth delivery modes, with inconsistent results across target behaviors and technology types. Nine studies compared eHealth to other modes of delivery (e.g. in person) with all finding no difference in SNAPO outcomes between groups at follow-up. This review provides some evidence for the efficacy of eHealth SNAPO interventions for young adults, particularly in the short-term and for alcohol interventions. But there is insufficient evidence for their efficacy in the longer-term, as well as which mode of delivery is most effective.