Contracts concluded by the farmer for the satisfaction of its professional needs form a family of contracts : all have part in the public action in agriculture. Their common identity is methodological. The contract is a legal instrument of the agricultural policy. All its utilities are mobilized for protecting the farmer or managing the agriculture. The special law of the agricultural contracts allows the penetration in the legal act of the value system chosen by the State. The contractual public order in agriculture also shows itself by an action of police of the contract. The contract in agriculture is not only an object of the public action : it is also one of its modalities. The contractualization shows itself at first by a joint production of the legal rule, with the State or between professionals. It shows itself then by the research for the consent in the standard, specially environmental. The contractual speech justifies the public action in agriculture. The agro-environmental contract allows the public remuneration for an ecological or territorial performance supplied by the farmer. This thesis brings to light a deep change of paradigm in the agricultural policy. While the law of the exploitation is absorbed, partly, by that of the company, an original legal corpus appears which proceeds of the new multifunctional identity recognized by the agricultural activity. This thesis also reveals the methodological sliding which takes place of the private contractual law forced by the public law towards the public law produced by contract.