Contracts are an economic tool used to arrange transactions which are not tradable in simple spot markets. This thesis focuses on the implications of different kinds of contracts to understand behavior in complicated interactions. The first part of this thesis focuses on explicit formal contracts that provide non-linear payoffs and examines theoretically and empirically the implications for effort and risk-taking. The second part of this thesis focuses in contrast on implicit contracts. Starting from a theoretical perspective about how implicit contracts for influence buying might work in a setting that precludes explicit contracts. This helps explains empirical puzzles as well as has new predictions. I then show empirical evidence consiste...