The notion of crowdsourcing existed long before the term itself was coined. The idea that small contributions from a group of individuals can be accumulated to accomplish some work or attain an objective has been observed in different realms of daily life for a few centuries. Over the years, crowdsourcing has emerged as a useful paradigm, showcasing the adage that "the whole is greater than the sum of its parts."The Web's emergence as a sociotechnical system has dramatically changed the scale and scope of crowdsourcing, opening the possibility of reaching crowds at an unprecedented global scale. Crowdsourcing has proven to be an increasingly important source of knowledge and data, as documented by prominent examples such as Wiki-pedia, where many authors contribute discrete and diverse information to form an authoritative reference knowledge base. The recent Wiki-data project (www.wikidata.org) and the popular ReCAPTCHA work by Luis von Ahn and colleagues [1] Particularly in research and science, crowd-sourcing has found noteworthy applications in solving real-world problems, ranging from intricate tasks such as protein folding and bio-molecule design [2] and mapping outer space [3] to aiding disaster relief initiatives [4] and assembling dictionaries. [5] © 2014 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. With the ubiquity of the Internet, and the concomitant accessibility of established microtask crowdsourcing platforms such as Amazon's Mechanical Turk (MTurk; www.mturk.com) and, more recently, CrowdFlower (www.crowdflower.com), researchers and practitioners are actively turning toward paid crowdsourcing to solve data-centric tasks that require human input. Popular applications include building ground truths, validating results, and curating data. These developments make it possible to build novel intelligent systems that combine the scalability of machines over large amounts of data with the power of human intelligence to solve complex tasks, such as audio transcription, language translation, and an-notation. For the rest of this article, crowdsourcing will refer to paid microtask crowdsourcing (wherein crowd workers receive monetary compensation for successfully completing a micro-task). For a thorough discussion of related terms and tasks in the context of human computation and collective intelligence, see the work of Alexander Quinn and Benjamin Bederson. [6] Owing to the diversity in the crowd in terms of workers' motivations, demographics, and competencies, both microtask design and quality control mechanisms play an unparalleled role in determining the effectiveness of crowdsourcing systems. [7] These two realms, which specifically are concerned with the requesters' perspecti...