How can one interpret Hawking radiation? The central point is that the notion of vacuum (and therefore also the notion of particles) loses its invariant meaning in the presence of a dynamical background. Incoming modes of the quantum field are redshifted while propagating through the collapsing geometry, which is why the quantum state of the outgoing modes is different. If the initial Towards a Full Quantum Theory of Black Holes 5 state is a vacuum state, the outgoing state contains “particles”. The redshift is especially high near the horizon, where the modes spend a long time before escaping to infinity. This is the reason why Hawking radiation is present very long after the collapse is finished for a comoving observer, contrary to what one would naively expect. The presence of the horizon is also responsible for the thermal nature of the radiation, since no particular information about the details of the collapse can enter. It turns out that the vacuum expectation value of the energy-momentum tensor of the quantum field is negative near the horizon, corresponding to a flux of negative energy into the hole (this is the basis for the pictorial interpretation of the Hawking effect, where one partner of a pair of virtual particles can fall into the hole, thus enabling the other partner to become real and escape to infinity, where it can be observed as Hawking radiation). For details of this scenario, I refer to e.g. Wipf (this volume), ’t Hooft (1996, and this volume), Birrell and Davies (1982), Wald (1994), and the references therein. The negativity of this expectation value is, like the Casimir effect, a genuine quantum feature. This negative energy flux leads to a decrease of the black hole mass and is equal to the positive flux of the Hawking radiation at infinity. From a simple application of Stefan-Boltzmann’s law