In this book we summarize important developments in telecommunications with a focus on Signalling System No. 7 (SS 7) network reliability. But why is SS 7 network security and reliability so important? According to the Open Systems Interconnection (OSI) model the term "network" refers to the Layer 3 network service, which is offered by the Message Transfer Part (MTP) and Signalling Connection Control Part (SCCP) to higher layers. An SS 7 network outage results in a breakdown of signalling between the individual nodes of the network, so that telecommunication network services are no longer available to customers. Apart from the tremendous nancial damage to the network operator, the social consequences of such a network breakdown cannot be underestimated. Since the MTP is the central part of the SS 7 network, it is of crucial importance for network security and reliability. The MTP is published in different versions and should now be what we call "stable". Is it really stable? Does it work properly in real networks? Does it t new requirements so that no further changes within the MTP are needed? Various different implementations have existed for a long time and are used in national as well as the international network. Due to this, an objective is not to p- form any further changes within the MTP in order to avoid compatibility problems with existing and running systems. Experience shows that compatibility problems are always related to further development of the MTP.