The 4th International Web Rule Symposium (RuleML 2010), co-located in Alexandria, Virginia, USA (near Washington, DC) with the 13th International BusinessRules Forum Conference2010,wasorganizedto meet colleaguesand to exchange ideas from all subareas of Web rule technology. The aims of RuleML 2010 were both to present new and interesting research results and to show s- cessfullydeployedrule-basedapplications. Thisannualsymposiumisthe?agship event of the Rule Markup Language (RuleML) Initiative. The RuleML Initiative (www. ruleml. org) is a non-pro?t umbrella organi- tion of several technical groups organized by representatives from academia, industry and public sectors working on rule technologies and applications. Its aim is to promote the study, research and application of rules in heterogeneous distributedenvironmentssuchastheWeb. RuleMLmaintainse?ectivelinkswith other major international societies and acts as an intermediary between various “specialized” rule vendor, application, industrial and academic research groups, as well as standardization e?orts including W3C, OMG and OASIS. After a series of successful international RuleML workshops and conferences, the RuleML symposia, held since 2007, constitute a new kind of event where the Web rules and logic community joins the established, practically oriented business rules community (www. businessrulesforum. com). The symposium s- ports the idea that there is a successful path from high-quality research results to deployed applications.