The aim of this thesis was to contribute to the development of empirical and theoretical knowledge about the legitimacy and comprehensibility of assessment- and decision-making processes within the sickness insurance system from a client perspective. The focus has been on interactions between clients and other stakeholders within the system, and the extent to which the clients understood and accepted the procedures. In addition, the legitimacy and comprehensibility of the system were studied through the lens of three theoretical concepts: social validity, social insurance literacy (SIL), and moral hazard. For this thesis, a mixed methods approach was adopted, consisting of one quantitative and three qualitative studies, including interviews, files, register data, and a questionnaire. Overall, the findings demonstrates that interactions between clients and other stakeholders were important both for the client’s sick-leave case and for their perceptions of sickness insurance procedures. A continuing dialogue between client and case manager tended to facilitate the client’s understanding and acceptance of the steps taken in the sick-leave process. Furthermore, a client’s perception of the justice of processes in the system was associated with the system’s ability to provide understandable and logical information and procedures. This thesis also demonstrates that assumptions of moral hazard were clearly present and that acts of power and mistrust occurred between stakeholders, as different stakeholders tried to influence the client’s sick-leave process. An acceptable and fair sick-leave process was described by the clients as being one that consisted of relevant procedures, was applicable to the unique client, and was a result of support from other stakeholders. In order to increase the comprehensibility, as well as the legitimacy and fairness, the authorities need to explain and justify a diverse range of steps beyond just the official decisions, so that clients are able to comprehend the what, how, and why of sickness insurance. In terms of SIL, clients’ abilities to obtain, understand, and act on information, did not influence whether they received sickness benefits or their perceived justice. On the other hand, higher scores of perceived justice was associated with high scores of perceived system comprehensibility and being granted sickness benefits. This indicates that what the system does, and what it does not do, influenced clients’ opinions of it, regardless of their own prerequisites.