Over the last two decades, mobile services rapidly grew from being an accessory service to becoming an essential part of day to day life, the colloquial saying that “one would never leave home without the house keys” is now the same for the mobile phone. This is reflected in the market penetration of mobile subscriptions over the last decades. Starting in 1991 when the first mobile phone call was made in Finland over the Global System of Mobile Communications (GSM), today we have reached over 4 billion mobile subscriptions, which is more than half of the world population. In comparison, there are only around 1.3 billion fixed line subscribers worldwide. Moreover, according to analytical estimation, the global subscriptions will reach 6 billion in 2014 and is estimated to achieve 100% global penetration by 2015, meaning that the number of subscriptions will almost match the world population. With this increase in the number of subscriptions and traffic, the energy and cost of mobile communications will increase as well. Mobile networks are already suffering high energy and financial costs. The energy bill accounts for a large portion of the operational expenditure (OPEX) of mobile telecom providers, and equals 1% of the monthly subscription income of mobile networks. Energy consumption has now become an important design metric for wireless systems, which before was confined to the familiar energy ratings seen on household appliances. The carbon footprint resulting from mobile phone usage has seen an alarming increase in the last decade with rise in mobile traffic, and the future is not looking rosy. In fact, the whole Information and Communications Technology (ICT) sector has been estimated to account for 2% of global CO2 emissions, a percentage comparable to the emissions due to global aviation. Mobile networks are estimated to contribute to around 0.2 of global CO2e (CO2 equivalent) emissions in 2007 and this is expected to increase to 0.4% in 2020. While the overall CO2e emission of ICT will less than double between 2007 and 2020, the Carbon footprint of mobile networks is predicted to almost triple during the same period, reaching more than one third of current annual emissions of the entire United Kingdom. Both the increase in cost and Carbon footprint of mobile networks mandate urgent solutions to the problem of energy consumption of mobile networks, especially if the EU is to meet the ambitious 20:20:20; a mandate set forth in order to reduce the greenhouse gas emissions and energy consumption by 20%, whilst increasing energy from renewable sources by 20%. This political leverage in unison with the expected forecast for higher mobile traffic demand has placed energy saving at the forefront of system design for telecom operators. Therefore disruptive solutions are needed to solve the problem of increased cost and energy consumption of future mobile networks: if we do not take radical action now then 4G users will be restricted to the nearest electricity socket providing a hypocritical stance on the 4G philosophy that preaches unrestricted freedom. This special issue intends to target some high quality research and practical initiatives in the hot field of green and J. Rodriguez (*) Instituto de Telecomunicacoes, Aveiro, Portugal e-mail: jonathan@av.it.pt