||We study a dilute Bose gas in a scaling limit where the Gross-Pitaevskii functional emerges. The derivation of the Bose-Einstein condensation in this case was first proved by Elliott H. Lieb and Robert Seiringer in 2002. In this talk, I will represent an alternative proof, which is based on (1) a combination of Dyson's lemma, (2) the quantum de Finetti theorem, and (3) a second moment estimate for ground states of the effective Dyson Hamiltonian. The proof applies equally well to the case where magnetic fields or rotation are present. This is joint work with Nicolas Rougerie and Robert Seiringer.