We are delighted to present this special issue of World Wide Web on Mobile Crowdsourcing. Recently, with the rapid development of mobile Internet and mobile social networking techniques, the scope of crowd problem-solving systems using mobile devices has been broadened and the traditional Internet Crowdsourcing is evolving into a new paradigm, i.e., Mobile Crowdsourcing (MCS), which facilitates the increasing number of mobile device users to participate crowdsourcing tasks. As a result, quite a number of crowdsourcing tasks that are difficult to complete based on Internet crowdsourcing has now become feasible, e.g., monitoring pollution level or noise level at the city-scale, predicting the arrival time of buses, collecting the truth happenings after a disaster, etc.