On the one hand, elementary particle physics relies on complex experiments conducted on both large and small scales. On the other hand, it is built upon a rich foundation of theoretical concepts, which span all areas of the field and are thus relevant to nearly every chapter of this book. Here we explore these concepts in more detail. The basis for describing all moving particles is the theory of special relativity. Here, we present concrete examples from everyday particle physics. Symmetries play a central role, particularly local symmetries, known as "gauge symmetries", which are essential for describing the forces in the Standard Model, the fundamental framework of elementary particle physics. Noether's theorem reveals a fascinating connection between symmetries and conservation laws, offering us an intuitive understanding of the reactions observed in particle experiments. Quantum field theory serves as the fundamental mathematical framework for our understanding of particle physics. It allows us to make predictions based on the Standard Model, which can then be tested through experiments. This forms the crucial link between the theoretical model and the observable world.