Unlike argumentation in safety cases, evidence (also an important component), has received less attention in the literature. In this paper, we compare the concept of evidence in different disciplines and analyse the characteristics of evidence in the safety domain. A model of evidence is proposed for a better understanding of evidence and its evaluation. Evidence assertions, the minimal assertions that are drawn out of the source data of evidence, are explained with the model. The model will provide clarity to the interface between evidence and argument that is useful for confidence establishment in safety cases.