Situations in which multiple parties with competing preferences interact are endemic throughout society. These essays consider the problem of a principal who seeks to induce self-interested agents to exert effort or promise contributions. The principal's task is to create an incentive structure that aligns the agents' preferences with his own. Chapter 1 considers a principal seeking to induce agents to exert costly, unobservable effort when the output they produce is unverifiable. The firm's solution is to hire multiple workers for some tasks and compare the output produced by the agents. The optimal mechanism bundles multiple tasks together to reduce the cost of monitoring and conditions wage payment for any task upon satisfactory completi...