In the recent past, the numbers of religiously- and politically-motivated terrorist attacks have increased, inevitably raising the question of effective measures to prevent further terrorist attacks. Empirical studies related to school shootings have shown that school shooters reliably (directly and indirectly) disclosed their intentions or plans prior to the attack, a phenomenon termed leaking or leakage. Leaking has been used for preventive purposes in this area of research. Recent research has indicated that leaking was also present prior to politically and religiously motivated terrorist attacks. In order to determine the current state of knowledge about leaking related to these offenses, we conducted a review of the international literature on religiously and politically motivated terrorist attacks. Up to 90% of the offenders showed some type of leaking prior to the attacks. A range of different forms of leaking could be observed. Leaking often occurred in the form of verbal communication with family and friends and/or via communication over the Internet. Terrorist offenders apparently tend to show leaking more often than other groups of mass murderers. Findings regarding similarities and dissimilarities in leaking between religiously motivated, jihadist and politically-motivated, far-right terrorist attacks were contradictory. We discuss the implications of these findings for practice and research as well as the strengths and possible weaknesses of the leaking concept.