Compensation for loss of income due to sickness from the Swedish national sickness insurance systems require that the capacity to work is reduced. The concept of working capacity, as well as how it should be measured and proven, has be a topic for discussion in research, and in law and politics, for many years. In this article, the burden of proof, medical certificates as evidence and evidential requirement regarding working capacity within the sickness insurance [sjukpenningförsäkring] are analysed and problematised from a legal perspective. Overall, the analysis shows that the burden of proof rests primarily on the insured, and that medical certificate must contain such information that it is possible to assess the consequences of a disease, which work task’s ability should be assessed against, and how the consequences affect the ability to work. The evidentiary requirement is “probable”, while there are exceptions that enable a lower evidentiary requirement, “consideration of the reasons”. In conclusion, it is most likely that the concept of working capacity, and how it should be understood and assessed, will continue to be the subject of ongoing discussions and investigations in the future.