**1. Introduction**

The concepts of Smart Grid and Smart City have rapidly expanded worldwide with the objective to raise sustainability standards, quality of life, and economic dynamism of future cities. At present, cities are responsible for more than 75% of waste, 70% of greenhouse gas emissions, and 75% of energy consumption [1]. This trend is going to be more and more significant due to the increase in population of urban areas. By 2045, according to the World Bank1 , the number of people living in cities will increase by 1.5 times to 6 billion. This fact emphasizes the importance of increasing sustainability, reducing waste energy, and 'smart' management of the available resources. In this context, natural gas systems, similarly to the rest of energy systems, are evolving towards a highly technified structure, where the consumer is called to play a starring role. The implementation of a natural gas smart infrastructure requires the proper development of smart metering and communication protocols to be able to give realtime and remote consumption readings and provide advanced services to the users.

<sup>1</sup> http://www.worldbank.org/en/topic/urbandevelopment.

The implementation of this smart architecture is ongoing in some European countries and abroad as will be discussed in the following sections.

Demand response programs are the best candidates to promote the consumers' flexibility, empowering the smart natural gas infrastructure to be a resilient and feasible energy network. Future trends in the development of natural gas systems will be focused on, exploring the huge potential that remains unexplored on the natural gas demand side, which could be used by gas network operators for the solution of technical constraints, balance services, or optimization of programming of underground storage. This potential is especially interesting at this moment, when the massive rollout of gas smart meters is taking place [2, 3]. In this context, smart gas systems would facilitate the use of demand response resources for the better operation of gas networks, similarly to how it happens in power systems.

The chapter will be organized as follows: at first, the state of the art of the massive rollout of smart meters currently ongoing worldwide will be presented. Then, demand response programs currently used in natural gas systems around the world will be explored and analyzed, including a review of some international experiences on this matter. Before, the novel application of smart grid concepts, exclusively used up to now for power systems, will be proposed for the architecture of the smart natural gas system, including the analysis of existing barriers that prevent the implementation of demand response programs in this kind of networks. Finally, the future developments required to optimize the use of this energy resource and to improve the operation of natural gas systems will be presented, identifying new agents and new energy resources (such as hydrogen) that are expected to contribute to developing the natural gas systems in the coming years.
