The discovery of the electron and its strange properties is the cornerstone of quantum mechanics. Based on their spin properties, photons and electrons, now categorized as fermions and bosons under the Standard Model, exhibit an enigmatic dual nature of wave and particle. This property, the interpretation of which is still debated today, is supported by various experiments, as we saw in Chap.  4 . One of these, in particular, can be considered the basis of any interpretative model on the dual nature of photons and electrons, the double-slit experiment. This experiment will lead to the famous Copenhagen interpretation, which is still a key to explain quantum mechanics today. With all its incredible weirdness, the double-slit experiment is thus the basis of quantum mechanics, as we shall see in this chapter. Experiments and interpretations of the strange quantum properties of the electron have also led to the discovery of other physical phenomena seemingly impossible according to classical physics, such as the tunnel effect and quantum entanglement. These phenomena have no classical equivalent and can be explained only by resorting to quantum properties of electrons.