Occasionally, reports are filed that suggest that TCAS II may not have performed as specified. That includes cases when no RA was triggered despite close proximity to the threat aircraft or an RA was triggered in the absence of any credible threat, the RA was inappropriate (e.g. towards rather than away from the threat), unusual RA sequences or technical anomalies. If it is found that TCAS II performed in such a way, investigations are typically carried out by Aircraft Operators, ATC authorities and, in particularly serious cases, by State investigation bodies. The purpose of these investigations is to find out what happened, establish whether TCAS II has performed as designed, look into all the contributing factors, and finally formulate recommendations in order to prevent similar events in the future. In this issue are described some real-life cases that have been reported and investigated. These investigations allow to establish what really happened in these events.