Even though different concepts as to the standard of proof have developed in the case law of the boards, they all have in common that a judgement is to be made on the basis of the application of the principle of free evaluation of evidence.

The EPO standard of proof is generally the balance of probabilities. By way of exception, the standard of proof of the balance of probabilities is shifted to a standard of proof beyond reasonable doubt mainly in opposition where only the opponent has access to information (evidence) concerning, for example, an alleged public prior use.

Of note in particular are some relatively recent decisions containing lengthy observations on the standard of proof and the previous case law on this, namely: T 2451/13 on the meaning of "beyond reasonable doubt" and T 545/08 on the meaning of "balance of probabilities" in the boards' case law; the latter decision, the board considered the matter in the general context of the law of evidence, concluding that a probability as low 51% would not suffice.