The social function of a public library is a factor intrinsic to its essence and values. The intent to positively influence the society surrounding the library has been present, in one way or another, since at least the origin of modern public reading, now around 150 years ago, with the aim of providing access to education for the public and of fostering democratic spirit (Faus, 2000). When we say 'in one way or another', we are referring to the fact that the concept of sociocultural action or intervention has varied over the years and that the assistentialist nature and initial benefit gradually gave rise to more participative and transformative practices. Even so, although this social vision is widely accepted in theory, libraries still struggle to define their planning around the social action that they put into practice. In this article, we will highlight the importance of the need to define social action in public libraries, from both a theoretical and a practical point of view. We will then analyze the Human Library as part of the planning and programming of services and social activities of public libraries, and, with a critical eye, attempt to pinpoint its strong points and the aspects which we consider to be its weaknesses. Finally, we will propose a series of reflections to bear in mind when producing a programme of social activities in a library, based on its being an accessible, participative and transformative space.