Incentive theory has been a major development in economics and the principal-agent model is the core of this theory. This authoritative collection brings together the essential literature concerning the principal-agent model when no restrictions on the design of the principal's contract exist in terms of complexity, enforcement and rationality. Part one covers the foundations of the principal-agent theory from the first historical formulation of the problem to the first attempts to formalize it. Part two deals with the case of moral hazard and adverse selection is the topic of Part three. Part four presents contributions on current research issues such as the impact of communication constraints, endogenous information structures and multidimensional incentive problems.