This doctoral dissertation investigates the more precise meanings of the duty of enquiry, the burden of proof and the evidentiary requirements of administrative authorities and administrative courts, and the relationships between these concepts within administrative procedures. As in any procedure, different interests come to light and must be balanced. Two interests of particular prominence in administrative procedures are the interest of materially correct decisions and the individual’s reliance upon the rule of law. These two interests impact on the meanings and extents of the duty of enquiry, the burden of proof and the evidentiary requirements, and how these are to be balanced. In certain types of cases, the interest of materially correct decisions is strong. In such cases, the duty of enquiry is extensive and the evidentiary requirement is limited. In other types of cases, the individual’s reliance upon the rule of law is prominent, and this leads to a limited duty of enquiry, with an extensive evidentiary requirement. The tension between these interests impacts upon the question of when a case has been sufficiently investigated that examination of the facts can begin. Other matters which are highlighted in this dissertation include when a reverse burden of proof may be used, which considerations are relevant in determining which evidentiary requirement applies and the more precise meanings of various evidentiary requirements.