Evidence law is the body of law that deals with how facts must be proved in a legal proceeding. In criminal law, the rules of evidence govern what is admissible or inadmissible in a trial and the procedures the police must follow when obtaining evidence such as interviews and items seized during a search. This also covers how witnesses are questioned during examination-in-chief and cross-examination and who is a competent and a compellable witness.
There are strict rules of evidence that apply to how witnesses are dealt with during a criminal proceeding, including rules about hearsay, opinion evidence, propensity evidence, relevance, privilege and identification evidence.
When parties to a criminal proceeding are in dispute about the admissiblity of an item of evidence, a preliminary proceeding called a voir dire may need to be held so that the court can make a ruling as to whether the evidence is to be admitted or excluded. While sometimes a piece of evidence will be clearly admissible or inadmissible, in other situations this may be less clear. It is also possible in some situations, for a court to exercise its discretion to exclude a piece of evidence that would ordinarily be admissible because of the circumstances under which it was obtained.