Quantum theory has found its way into almost all areas of physics and serves as a fundamental starting point for many fields. Therefore, knowledge of it is essential for anyone engaged in or interested in physics. This presentation focuses on the basic ideas of quantum theory and its implications for physics as a whole. It also aims to practice the handling of quantum theoretical methods to the extent that a study of the literature in various specialized fields, such as atomic and molecular physics, chemistry, solid-state physics, or high-energy physics, can be directly pursued. Despite its relatively small scope, a certain overview of quantum theory is also sought for those who can only engage with it intensively once, such as many science teachers.