We characterize optimal reward-based crowdfunding where production is contingent on an aggregate funding threshold. Crowdfunding adapts project-implementation to demand (market-testing) and its multiple prices enhance rent-extraction via pivotality, even for large crowds, indeed for arbitrarily large if tastes are correlated. Adaptation raises welfare and rent-extraction can enhance adaptation, but sometimes distorts production and lowers welfare. Threshold commitment, central to All-Or-Nothing platforms, raises profits but can lower consumer welfare. When new buyers arrive ex-post, crowdfunding’s market-test complements traditional finance and informs subsequent pricing. We prove that crowdfunding is a general optimal mechanism in our baseline.