This article is dedicated to the 100th anniversary of quantum mechanics and, in particular, electron spin — a phenomenon without analog in classical physics. The electron spin hypothesis was proposed by Uhlenbeck and Goudsmit in 1925—1926 to explain numerous experimental data. Two years later, Dirac discovered the existence of electron spin while constructing a relativistic wave equation for a free particle. This came as a great surprise to Dirac, since, as he later claimed (in 1977), his goal was to obtain a relativistic wave equation for the simplest particle, which he considered to be a particle with zero spin. It turned out that the simplest particle is a particle with spin 1/2.