This workshop is intended for people who are interested in both machine learning and web search. With its tens of billions of unstructured and dynamic pages and its increasing number of users, the World Wide Web poses new great challenges to the existing machine learning algorithms, and at the same time it also fuels the rapid development of new machine learning techniques. This workshop aims at bringing machine learning and web search people together to discuss the fundamental issues in web search from relevance ranking and web spam detection to online advertising. The topics will be mainly focused on new web page ranking algorithms and also online advertising related issues, like click-rate prediction and content matching.