Smart worlds begin with smart things, such as smart objects, smart cities, smart manufacturing, and smart systems, are overlaid with sensing and actuation, many embedded in things, and eventually encompass all aspects of the cyber, physical, social, and thinking hyperspace. In the future, human beings will live in a smart environment where both life and work are well addressed by technology, whereas humans will be responsible only for providing creativity. For this purpose, we have organized the first 2015 Smart World Congress, including five IEEE International Conferences. Specifically, a variety of challenges are presented in the field of smart world. Therefore, an open forum on the top ten challenges (Top10Cs) for smart worlds was held under the congress to identify the main challenges through collecting the intelligence existing particularly in crowd wisdom. Top10Cs and related works, as a crowdsourcing approach, discuss and analyze the top challenges for smart worlds based on the selective results of the crowd and experts via an online platform. Moreover, we summarize the experiences obtained in organizing the Top10Cs forum. INDEX TERMS Smart worlds, top challenges, collective intelligence, crowdsourcing, crowd wisdom.