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With the ever-increasing trend in the number of social interactions getting intermediated by technology (the world wide web) as the backdrop, this thesis focuses on the design of mechanisms for online communities (crowds) which strive to come together, albeit in ad-hoc fashion, to achieve a social objective. Two examples of such web-based social communities which are of central concern in this thesis are crowdsourcing markets and crowdfunding platforms. For these settings which involve strategic human agents, we design mechanisms that incentivize contributions (effort, funds, or information) from the crowd and aggregate these contributions to achieve the specified objective. Our work is thus concerned with the challenge of designing mechanisms which encourage social coordination and cooperation among large groups of (of-ten) anonymous users to achieve group efficiency (social welfare). We address the following four related challenges: • Can we design mechanisms to solve the free-rider problem in public goods settings? Can large anonymous groups of individuals be incentivized to contribute to create public goods? • Can we design mechanisms that harness social networks to achieve coordination of contributions towards a public good to ensure that publicly desirable goods are successfully funded via private contributions? How do we make such mechanisms fair • Can we design mechanisms that improve the efficiency of markets by expanding the set of individuals who participate in the market? Can these individuals be incentivized to increase the group efficiency and, if so, at what cost? • Can we design mechanisms that make crowdsourcing markets more equitable by offering participation opportunities to novices and higher incentives to agents with high reliability? What is the price of reliability? Using mechanism design as the principal design tool, the thesis attempts to offer rigorous solutions to the above challenges