## 1 Introduction  
The impressive and fast development of the biodiesel industries has led to a large surplus of glycerol since about 100 kg of this byproduct are formed per ton of biodiesel (Scheme 1). According to recent estimates [1], world-wide production of glycerol could have reached 2 million tons in 2010 and it is expected to grow in the near future due to the increasing demand for biodiesel, as well as the emergence of other large-scale processes based on the conversion of cellulose and lignocellulosic in which glycerol is also a byproduct. Consequently, glycerol is nowadays in urgent requirement of exploitation [2]. Most efforts in this area are focusing on the development of procedures to convert glycerol into value-added chemicals, which have either been previously prepared from petrochemical sources or suffered from environmental problems during their preparation [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12]. Glycerol reforming for hydrogen production [13; 14; 15] and its transformation into fuel additives [16] are also active areas of research. **Scheme 1. ** Overall reaction in the production of biodiesel. Replacing of hazardous chemicals by more environmentally friendly alternatives is also a matter of current interest, in line with the philosophy of Green Chemistry [17; 18]. In this context, the introduction of ecofriendly solvents is a key area [19; 20; 21; 22] because solvents account for 80%-90% of mass utilization in a typical pharmaceutical/fine chemical operational process, and they are therefore responsible for most of the waste generated in the chemical industries and laboratories [23]. Nowadays, most solvents employed are substances derived from fossil fuels and have a negative impact on the health and the environment. In addition to water [24; 25; 26; 27], biomass-derived chemicals are probably the most promising alternatives to replace these harmful solvents [19; 20; 21; 22]. Its renewable origin and unique combination of physicochemical properties, such as high polarity, low toxicity and flammability, high boiling point, ability to form strong hydrogen bonds and to dissolve both organic and inorganic compounds (salts, acids, bases and transition metal complexes), make glycerol a good candidate to be employed as green reaction medium for synthetic chemistry [28]. This innovative way to revalorize glycerol has been very well received by the chemical community and a wide variety of organic reactions and synthetic methodologies have been successfully developed in this medium in recent years [29; 30; 31; 32; 33]. Remarkably, in some cases, the use of glycerol as solvent was found to enhance the effectiveness [34] and selectivity [35] of the reactions, and allowed easy product separation [36] and catalyst recycling [37]. Recent works have demonstrated that glycerol can also act both as solvent and hydrogen donor in metal-catalyzed transfer hydrogenation (TH) reactions. TH is recognized as an efficient reduction method of unsaturated organic compounds (ketones, aldehydes, amines, olefins, _etc._) and constitutes a safer alternative to the classical catalytic hydrogations using highly flammable molecular hydrogen [38; 39; 40; 41; 42]. Replacing the most commonly employed hydrogen sources in TH reactions, _i.e._, 2-propanol or formic acid/triethylamine mixtures, by glycerol results not only economically appealing but also more environmentally friendly. Interest in TH processes from glycerol relates also with the concomitant formation of dihydroxyacetone (Scheme 2), the expected oxidized form of glycerol according to the primary and secondary alcohol oxidation potentials [43], due to the industrial relevance of this chemical [44]. The aim of the present review article is to provide a comprehensive account of the advances reached in the use of glycerol as solvent and hydrogen source in metal-catalyzed transfer hydrogenation of organic compounds. The utility of glycerol as solvent and reducing agent for the generation of metal nanoparticles is also highlighted. **Scheme 2.