For over a decade the Ericsson SGSN/MME, in large parts based on Erlang, has been the market leading product in its segment. The SGSN/MME is an integral part of the packet switched telecom network (commonly known as "mobile broadband"), and basically serves as a big router in the core network providing mobility and session handling for mobile phones. Today the same system can handle 2G, 3G and 4G traffic at the same time (one SGSN/MME can serve several Radio Access Networks). In this talk we will try to show how Erlang has been a central part in the construction and evolution of this system over a period of almost 15 years. The SGSN/MME is still being actively developed; 4G and completely new hardware being only two of the recent additions. In our opinion the early choice of Erlang for this product has been one of the key reasons for its big success. We will try to demonstrate this with examples of how Erlang core features are utilized in the systems design. The SGSN/MME code base is huge - the Erlang parts alone are well over a million lines of code, and the C/C++ parts several times bigger. This in itself becomes a challenge to handle in a system that has been developed for so long and with a very large number of people involved.