Public Interest Immunity—previously known as Crown privilege—is an exclusionary rule of evidence. When it applies, it excludes relevant evidence (usually but not exclusively documents) from production in court. Thus when the rule applies the public interest in the due administration of justice has given way to a greater public interest that calls for the evidence in question to remain secret. It has been much discussed recently, not least in the Scott Report which made several suggestions for reform and, more significantly, revealed in detail how PII claims were used by the government in litigation. The government has responded by announcing important reforms. A review of developments is thus appropriate.