The objectives of this systematic review are to evaluate the effectiveness of web-based interventions on weight loss and maintenance and identify which components of web-based interventions are associated with greater weight change and low attrition rates. A literature search from 1995 to April 2008 was conducted. Studies were eligible for inclusion if: participants were aged ≥18 years with a body mass index ≥25, at least one study arm involved a web-based intervention with the primary aim of weight loss or maintenance, and reported weight-related outcomes. Eighteen studies met the inclusion criteria. Thirteen studies aimed to achieve weight loss, and five focused on weight maintenance. Heterogeneity was evident among the studies with seven research questions examined across interventions of varying intensity. Seven studies were assessed for effectiveness based on percentage weight change, with four studies deemed effective. Although the four meta-analyses suggest meaningful weight change, it is not possible to determine the effectiveness of web-based interventions in achieving weight loss or maintenance due to heterogeneity of designs and thus the small number of comparable studies. Higher usage of website features may be associated with positive weight change, but we do not know what features improve this effect or reduce attrition.