There are two possible ways to understand form and substance in legal reasoning. The first refers to the distinction between concepts and their applications, whereas the second concentrates on the difference between authoritative and non-authoritative reasons. These approaches refer to the formalistic and positivistic conceptions of the law, the latter being the author's point of departure. Nevertheless, they are both helpful means of analysis in legal interpretation. Interpretation is divided into formal and substantive justification. They have certain functions and they are utilized in interaction. Authoritative reasons and formal reasoning constitute the necessary point of departure. However, substantive reasons are also necessary in order to justify choices included in interpretation. In addition to formal and substantive reasoning, the role of legal concepts is analysed.