What is social capital and how is it defined? “Social capital” has been introduced as a concept capable of linking together the human sciences. Social capital has been defined as the profits of network co-operation, that is, as a resource in line with financial and human capital. Cross-disciplinary human science research has shown that social capital can be accumulated both at micro- and macro-levels. This article analyses the various definitions and applications of the concept. Taking Bourdieu’s definition as a starting point, it shows how the concept has been elaborated by American sociologists and political scientists.