The analysis of the singularities in the S-matrix of particles has shown that perturbative structure field theories can be much simpler than what Feynman diagrams tell us. In particular, there exist classes of theories whose perturbative S-matrix is completely determined by a sub-set of its singularities. At tree level this reflect with the existence of recursion relations. At loop level, there are special singularities, the leading singularities, which are believed to determine a class of theories which N=4 SYM and N=8 supergravity belongs to. I will discuss both the tree and loop level, with a particular attention on the leading singularity technique and its application to the investigation of the perturbative N=8 supergravity.