It is the second monograph by Carlo Smuraglia, published in 1963 by Giuffrè, and perhaps his least known work. Nonetheless, it remains to this day one of the very few studies dedicated to the topic and, despite the numerous major reforms that have changed the face of labor law in the years since its publication, it retains elements of relevance that cannot be overlooked even today. The fundamental thesis is that the existence of an agreement cannot be determined based on the presumption of acceptance of a particular treatment. From the receipt of a salary or a change in the relationship, writes Carlo Smuraglia, the only legal certainty that can be deduced is that of the receipt or application of the new working conditions. It is perhaps in terms of the demands for flexibility in job performance and the varied galaxy of forms of availability required of the worker that the issue of interpreting the worker's conclusive behavior seems to take on even more significance today. But the principles and hermeneutic criteria that Carlo Smuraglia had developed back in 1963 should today be reevaluated also in light of the erosion of job stability that occurred with the reforms of the dismissal regulations in 2012 and 2015.