This chapter explains the financial meaning and importance of Socially Responsible Investment (SRI), also called ethical investment. Currently, SRI is a private initiative to invest increasing flows of financial resources in environmentally and socially sustainable activities and to invest nothing in anti-ethical projects. Main SRI agents are banks and institutional investors who are engaged in policies such as sustainable consumption of energy and natural resources, ecosystem protection, advanced medical projects, technological research, education of young entrepreneurs, anti-tobacco campaigns, safety and healthcare in the workplace, and others. These agents think that traditional financial criteria such as profitability and risk should be combined with SRI criteria to select stock portfolios. In SRI, Multiple Criteria Decision Making (MCDM) approaches seem to be helpful as several criteria, not only financial but also environmental, social or governance concerns are taken into account. Regarding MCDM, a brief overview is included in this chapter to introduce this methodology into SRI decision problems.