ORSUM Chairs' Welcome & Organization.

It is our great pleasure to welcome you to the WWW 2018 Workshop on Online Recommender Systems and User Modeling (ORSUM). We have received eleven submissions covering highly relevant topics in the research related to recommender systems and user modeling. During this workshop its participants will have the opportunity to see eight presentations corresponding to the accepted papers and to discuss the recent advances on these topics both with authors and other researchers in the audience. The topics of the talks include, among others, location and news recommendation, local models for online recommendations, recommendations' diversity, page optimization, context-aware recommender systems, crowdsourcing and incremental matrix factorization methods.