The standard of proof is the degree of confidence or level of certainty that a judge / arbitrator needs to attain in order to establish the existence of a fact. In Common Law legal systems the applicable standard of proof is ostensibly objective and the notion is well developed in case law and academic writings. Conversely, in civil law systems the standard of proof is scarcely ever discussed and judges are expected to rely on their own inner conviction as to whether facts have been established with sufficient certainty. It is, however, unclear the extent to which these differences in the articulation of standards of proof make any difference in practice. In international arbitration, the applicable standard of proof is a nebulous concept. There are different views as to whether it is regulated by the substantive law or arbitral procedural law and just how much discretion an arbitral tribunal has or should have in applying it. Whatever the appropriate standard of proof for arbitration may be, it is an insufficiently developed subject that parties and arbitral tribunals alike should pay more attention to, particularly given its potential to impact significantly on the substantive outcome of claims.