## 1 Introduction  
According to the European Union (EU) Renewable Energy Directive [1], the production of energy from renewable sources is becoming an urgent target to reduce the impact of greenhouse gases (GHG), mainly derived from fossil fuel combustion [2]. This directive by establishing an overall policy for the production and promotion of energy from renewable sources in EU, requires the EU to fulfil at least 20% of its total energy needs with renewables by 2020, to be achieved through the attainment of individual national targets. In addition, all EU countries have to warrant that at least 10% of their transport fuels come from renewable sources by 2020. On 30 November 2016, the Commission published a proposal for a revised Renewable Energy Directive to make the EU a global leader in renewable energy production and use by meeting the target of at least 27% renewables in energy consumption in the EU by 2030. The proposal specifies national renewable energy targets for each country, which define how they plan to meet these targets by national renewable energy action plans.  
Among non-conventional energy resources (e.g., solar, wind, hydro-wave, geothermal and biomass) offering interesting opportunities as they are unlimited and cheap [3], biomass-produced biogas is able to provide high quantity of energy with significant Greenhouse gases (GHG) savings,as compared with fossil fuels [4]. According to the European Directive 2009/28/EC, the term biomass designates \"the biological origin and biodegradable fraction of products, waste and residues from agriculture (including vegetal and animal substances), forestry and related industries including fisheries and aquaculture, as well as the biodegradable fraction of industrial and municipal waste\". Europe and Central Asia generated 392 million tons of waste in 2016, and about 31 percent of waste materials is currently being recovered through recycling and composting [5]. Thus, the development of a circular economy (CE) using innovative technologies to reuse and convert agro-food waste, may represent a multipurpose achievement in terms of renewable energy production and waste reduction. According to [6], biomass conversion may be achieved by thermochemical conversion (used for dry wood residues with C/N\(>\) 30 and humidity less than 30%), combustion (occurring in presence of oxygen with carbon oxidation), pyrolysis (used for biomass with less than 15% humidity), biological conversion, including alcoholic fermentation and anaerobic digestion (used for materials with a C/N ratio \(<\) 30 and a humidity \(>\) 30%). In particular, the anaerobic digestion (AD) of biomass provides a versatile renewable source of energy, the biogas, useful as a substitute for fossil fuels for both power and heat production [7]. The EU sustainability criteria have been extended to cover solid biomass and biogas used in large heat and power plants (above 20 MW fuel capacity) with a reduction in GHG emissions from biomass-based electricity and heat production to a level at least 80% and 85% lower than fossil fuels by 2021 and 2026, respectively.  
This review focuses on the production of biogas obtained through AD from residual agro-food biomasses, analyzing the strengths and weaknesses of the different types of substrates and of plant technologies, as well as the most promising tools to improve AD efficiency, with the aim to identify the best process conditions.