Authentication or identification of real evidence (i.e., tangible evidence that is historically connected with a criminal case and not merely illustrative) refers to the requirement that an item of evidence be proved to be genuine, that it is what its proponent claims it to be. Sometimes the condition of an object is as important as its identity. Alteration of the item may reduce or negate its probative value and may mislead the jury. Thus, before physical objects are admissible in evidence the proponent must establish that they are in "substantially the same condition as when the crime was committed." Determining what changes are "substantial" depends on how the changes affect the relevance of the evidence. Proving the identity of real evidence can be done by establishing that it is readily identifiable (e.g., it bears serial numbers, police markings, natural marks) or by establishing a "chain of custody." The article describes the components of a chain of custody and reasons why one might be necessary, and examines relevant federal and state rules. Notes