This dissertation studies the circumstances under which explicit norms of evidentiary admissibility were adopted in twelfth- and thirteenth-century Roman-canon procedure. I demonstrate that norms of admissibility arose in two areas: the law of positions and the law of witnesses. I argue that these norms first emerged in Roman-canon procedure to mitigate the problems caused by shifting a measure of control over examination of parties and witnesses from the adjudicator to the parties themselves. I focus in particular on the problem of parties' abusive examination of their opponents and the problem of parties' tendency to produce evidence that did not serve the fact finder’s informational needs.