The notion of social capital is probably the most successful product introduced by sociology into the market of ideas in recent years. However, the rapid spread of the concept risks diluting its content and thus reduce its analytical efficacy; as the literature has highlighted, the notion of social capital is by now applied to so many things that it has lost any distinct meaning. The article seeks to give precise definition to the notion of social capital, tying it to a particular model of social relation: that of social exchange. It develops the idea (typical of social exchange theory) of social approval-recognition as a generalized means of exchange analogous to money, which functions as a deposit of value; a medium that makes it possible to accumulate resources - a capital - in social relations which can also be used in the future. The second part of the paper develops the macro dimension of the theory, considering associations as particular forms of social capital.