I am going to highlight recent developments in cosmological
perturbation theory, at linear order and beyond. After a brief
introduction of perturbation theory at linear order, I will show
similarities to and differences with the theory at second order. The
talk continues with the discussion of the entropy or non-adiabatic
pressure perturbation and finishes with two applications of second
order perturbation theory, namely the generation of vorticity and
magnetic fields.