One of the most elusive scientific challenges for over 150 years has been to explain why cooperation survives despite being a seemingly inferior strategy from an evolutionary point of view. Over the years, various theoretical scenarios aimed at solving the evolutionary puzzle of cooperation have been proposed, eventually identifying several cooperation-promoting mechanisms: kin selection, direct reciprocity, indirect reciprocity, network reciprocity, and group selection. In this talk, we discuss the results of several experiments that were aimed at answering the previous long-standing puzzle, sometimes ruling out some of the theoretical hypotheses and others proposing new mechanisms so far less explored. We conclude by discussing what kind of questions remain and possible ways to tackle them.