The intellectual performance of professional consists in an activity carried out with full autonomy and discretion on a subject - the professional - in respect of his client - a creditor of intellectual performance - and it's a topic that always turns on the legal debates , both with reference to the nature and characteristics of the performance, both with reference to the contractual liability in the event of non-fulfillment of the obligation. This type of performance has always been the subject of a debate on the legal doctrine focus on the legal classification and is the center of the "classical" dichotomy “duty of best efforts” and “duty to achieve a certain result”. This distinction in today's legal debate has perhaps lost consistency but which is still very dear to the case law, in particular with reference to the obligation of the lawyer. If, in fact, with reference to the obligation of doctor the case law has brought forth a work of demolition of the dichotomy in question, with reference to the performance of the lawyer you will find many judgments which continue to give importance and to emphasize the differences that are classically traced to dichotomy “duty of best efforts” and “duty to achieve a certain result”, especially in order to the burden of proof in cases of breach of contract. This thesis analyzes profusely issues relating to the nature of the obligations of the professional and intellectual work, and the debate on the dichotomy “duty of best efforts” and “duty to achieve a certain result”, as well as the overall theme of the failure, then dropped the details of the events doctrinal and jurisprudential which involved the allocation of the burden of proof of the failure, particularly as regards the obligation of the contract for project work.