Semi-classical physics uses classical periodic orbits to compute the quantum behavior of a system. It describes very efficiently billiards and 1-well potentials, but fails whenever quantum tunnelling plays a role. One way to solve this problem is to use complex orbits which "tunnel through the barrier". In this seminar, I will briefly specify what are these orbits and their role in multi-dimensionnal tunnelling. As an illustration, I will consider the energy splittings in 1-D and 2-D double-well potentials.