The first IEEE International Workshop on Software Defined 5G Networks (Soft5G 2015) will be held on April 17, 2015 in London, U.K. at the Cruciform Building of University College London (UCL) along with 1st IEEE International Conferenceon Network Softwarization (NetSoft 2015). Software defined networking (SDN) is undoubtedly one of the trending topic in the wired and the wireless networking domains raising interested among the academic and the industrial communities alike. Additionally the evolution of Network Functions Virtualisation (NFV) brings the opportunity to implement network functions in software on top of generic hardware architectures. In parallel, network related standardization activities are mushrooming across the most important SDOs. The mobile networks beyond LTE/EPC are currently required to cope with dramatic increases in data traffic and with a highly diverse devices, applications and services ecosystem. As a result, several research initiatives on 5G mobile networks have been launched worldwide targeting the definition of this next generation mobile networking ecosystem. SDN brings the promise of enabling flexible, scalable, highly configurable and reliable network functions and complete solutions for future 5G mobile networks, by leveraging the programmability advantages in the network control and management planes. This workshop aims at bringing together research and industry partners to present and discuss preliminary research results, ongoing work and experiences on the topic of software based, programmable 5G networks and services.