An offer of proof—also known as an informal bill of exception—is a procedure available during trial for the presentation of evidence the trial court has excluded. An offer of proof is made outside of the jury’s presence and often after the judge has sustained an objection to the admissibility of the evidence.

A bill of exception—also known as a formal bill of exception—is a procedure that allows a party to complain on appeal about evidence that would not otherwise appear in the trial record (because there was no offer of proof made at trial) by detailing the evidence (testimony, etc.) that would have been offered if the trial court had allowed it.

In Pennsylvania, an offer of proof is a procedural mechanism used during a trial when the court excludes evidence. This process allows the attorney to create a record of the evidence that they believe should be admitted. The offer of proof is typically made outside the presence of the jury, following a judge's ruling that sustains an objection to the evidence's admissibility. The purpose is to preserve the record for appellate review by showing the appellate court what the evidence would have been and allowing the court to assess whether excluding it was harmful error. On the other hand, a formal bill of exception, which is less commonly used in modern practice due to changes in rules of procedure, is a written statement by the judge detailing the evidence that was not allowed, which the attorney believes should have been admitted. This is used to ensure that the appellate court is aware of the excluded evidence even if an offer of proof was not made during the trial. Pennsylvania's appellate procedure rules govern these processes and dictate how and when these actions must be taken to preserve issues for appeal.