Inter-domain routing security is a big actor in end-to-end network connectivity. The protocol currently implemented was not designed to cover such a critical aspect, and so many vulnerabilities crop up, having a strong impact on the whole system. Despite much effort in the past focusing in addressing security issues, no solutions have become a reality, hence novel solutions must be sought to reduce the vulnerabilities space. The Border Gateway Protocol (BGP) is a critical component of the Internet's infrastructure used as the de facto inter-domain routing protocol among autonomous systems. It was conceived without an internal security mechanism and hence is prone to a number of vulnerabilities and attacks, which have resulted in partial paralysis of the Internet. Thus, securing BGP has been an active research area for almost a decade now. Several strategies, ranging from complete replacement of BGP to addition of new features in it, were proposed for the purpose of security but none of them were pragmatic enough to be adopted. Recently, the Secure Inter-Domain Routing (SIDR) working group of the IETF has put forward a set of recommendations which seem promising to some extent. This book introduces the reader to the main concepts in inter-domain security, reviewing the most significant contributions and also introducing the current efforts being developed by the scientific community to deal with the overall weaknesses and limitations that still exist.