A fundamental goal of physics is to describe and understand all the forces appearing in nature. All the phenomena occurring in the universe can be traced to the action of just four fundamental forces or interactions, namely, the gravitational, electromagnetic, strong, and weak interactions. This classification, although it greatly simplifies the picture of different interactions, does not provide real understanding of the nature of the different forces. Recent theoretical work suggests the existence of an M-theory, which would unify and explain all the interactions in a natural way.