This thesis focuses on presenteeism at work, a phenomenon that refers to the behavior of employees who go to work although they are sick. Our goal is to try to answer the following question: what is the decision process that leads an individual who is temporarily affected by sickness to decide to report sick and/or to attend despite the illness? Specifically, our goal is to study the determinants that lead a sick employee to choose between three options: a total absence (the employee is absent for the whole duration of the spell of illness), total attendance (he works for the whole duration), partial attendance, that is to say a combination of absence and attendance in the same spell of illness. To our knowledge, this third possibility has never been considered in the literature, which only distinguishes absence and attendance. To analyze the choice between these three possibilities, we have designed and conducted a survey to identify, for each employee in the survey, his last spell of illness, and for it, the decision between total absence, total attendance and partial attendance. As we will show, taking into account combinations of absence and attendance in the same spell of illness significantly modifies the measure of presenteeism, in comparison to the way it is generally measured in the empirical literature. Our results also highlight the predominant role of illness characteristics in the behavior of the employee, especially the role of the duration of the spell of an illness. Our results also show the effect on the decision of, one the one hand, risks that the employee takes for his future health if he decides to attend, totally or partially and, on the other hand, risks he takes for his job and for his future wage if he decides to report sick leave during the full spell of illness or during just a part of it ; Cette thèse porte sur le présentéisme au travail, phénomène qui désigne le comportement des salariés qui se rendent à leur travail alors qu’ils sont malades. Notre objectif est de tenter de répondre ...