At the Golden Age of Differential Topology, for every dimension, the question of exotic spheres arose. A sub-question emerged: does there exist in dimension n an exotic sphere just obtained by gluing two n-balls along their boundaries? Such spheres, up to diffeomorphism, form a group for the connected sum denoted by \(\Gamma _n\) . The first discovery of this type of exotic spheres goes back to J. Milnor (1956): he proved that \(\Gamma _7\) is not trivial. By proving \(\Gamma _4=0\) , J. Cerf (1968) stated that the gluing of two four-dimensional balls always gives rise to the standard \(S^4\) . Actually, Cerf proved that every diffeomorphism of \(S^3\) is isotopic to a linear diffeomorphism. In this chapter we present a foliated proof of Cerf’s theorem.