Casimir physics, or the physics of the quantum vacuum, contains a host of puzzling phenomena, and the potential for significant applications to nanomachinery. Its reach goes from the microscopic
structure of matter to the structure of the cosmos. In this talk, I will discuss some of these phenomena, and how they can be understood from the point of view of interactions (van der Waals or CasimirPolder) between neutral, polarizable, atoms. Such an atom can experience friction when moving through vacuum, the Einstein-Hopf effect, which recently has been generalized to the relativistic regime, where the temperature of the atom can be different from that of the vacuum. I will discuss how such friction might be observed. Summing van der Waals forces between atoms, taking into account the finiteness of the speed of light (retardation), results in the Casimir force between metallic and dielectric bodies, which forces can be either attractive or repulsive. This summation can also be done for the constituents of a single body, yielding the self-free energy and entropy, which can be negative. The meaning of that surprising sign is explained.