Introduction. The purpose of Chapter 6 is to discuss from a general point of view the dressing of a source by the vacuum fluctuations of the field coupled to the source. In Section 6.1 we show that in quantum optics, as well as in different branches of physics, virtual quanta of the field are present in the ground state of the source-field system. Three examples are considered: a two-level atom coupled to the vacuum electromagnetic field, a static model of a nucleon coupled to the vacuum meson field and an electron coupled to the optical phonon modes of a semiconductor (Fröhlich polaron). Section 6.2 is dedicated to a qualitative discussion of the physical nature of these virtual quanta and of their spatial distribution around the source. The dressed source is then defined as the bare source together with the virtual quanta surrounding it. This virtual cloud is shown is Section 6.3 to lead to a change of the energy levels of a nonrelativistic free electron interacting with the vacuum electromagnetic field. This kind of self-energy effect can be represented by a mass renormalization of the free electron. Self-energy effects due to the virtual cloud are discussed in Section 6.4 for each of the three examples of dressed sources considered in Section 6.1.