Before delving into the main content of this book, let us first review some basic concepts that will be essential throughout. At the LHC, protons in the accelerator and the newly produced particles typically travel at speeds very close to the speed of light. To understand their behavior, we must draw on the principles of special relativity. In particle physics, we work at the subatomic level, so it is crucial to understand key concepts of quantum mechanics. This includes how we can make accurate predictions despite the inherent randomness of individual events, such as in radioactive decay. We also explore the concept of spin, a fundamental property of particles that describes their intrinsic angular momentum. Additionally, we discuss how particles can be created, destroyed, and even decay spontaneously. In order to learn something about these unimaginably small particles, we carry out huge scattering experiments. What are these particles and why are we performing these experiments? And how can we use relatively simple graphical means, the so-called Feynman graphs, to illustrate and explain the reactions of elementary particles? Finally, we examine the crucial role that symmetries play in understanding particle physics—even when they are only approximate or have been broken!