This article is an introduction to the field of highly available databases (HADBs), especially in the context of embedded systems. The ground work done at the Service Availability Forum is also presented. The required availability level of contemporary telecom systems is around “5 nines”, that is 99.999 % availability. This, combined with a limit on the duration of MTTR (service downtime) in the range of single seconds, constitutes a serious challenge for any system. A database management system, as a component of a 5-nines system has to deliver even better availability while maintaining the integrity of the data over any failure scenario. Various approaches to HADBs are presented. An overview of basic commercially available architectures is given, together with various trade-offs achieved with the technology. A case study of applying an HADB to a telecom system is presented. The criteria for and behavior of an HADB compliant with the SA Forum’s specifications is described. Finally, benefits and the prospects of the HADB technology are summarized.