Challenges Track Chairs' Welcome.

It is our great pleasure to welcome you to the WWW 2018 Challenges Track. It is the first time that the WWW conference includes such a track, which aim was to showcase the maturity of the state of the art on tasks common to the Web community and adjacent academic communities, in a controlled setting of rigorous evaluation. Through our call for challenge organisation, we also wanted to see which open questions might be seen as most relevant in this community today, and how groups of researchers might come together around shared resources (e.g. datasets) to address those questions in a hands-on, practical way. We received five proposals for challenges, and selected four of them, which were proposed by well established researchers in their respective domains. The topics addressed varied from being purely focused on a domain specific task, without constraints on the technical approach to take (e.g. music genre recognition) to fundamentally technical tasks, which can be seen as useful across domains (question answering), with two of the challenges representing a mix of both.