The so-called Dirac sea, an infinity of electrons filling up all negative energy states in the world, was the most common conception of the vacuum and anti-matter during the 1930s. In this chapter, I will review the development of this conception with specific focus on the role of virtual transitions (modern terminology). According to the argument developed in the first four sections, it was the occurrence of negative energy states as virtual ones that led Paul Dirac to a reconsideration of the negative energy states in general terms. In the further development and as my argument will go, the description of the scattering mechanism pointed to conceptual inadequacies of Dirac’s initial description of a hole in the sea as a proton. This chapter will thus discuss the impact of virtual entities on the interpretation and development of the conceptual framework of quantum electrodynamics. Furthermore, I will describe one instance of what I called a “false prediction” and discuss the relation of Dirac’s reasoning to arguments brought forward in the realm of Beyond Standard Model Physics today.