Relativistic quantum chemistry is the relativistic formulation of quantum mechanics applied to many-electron systems, that is to atoms, molecules and solids. It combines the principles of special relativity, which are obeyed by any fundamental physical theory, with the basic rules of quantum mechanics. By construction, it represents the most fundamental theory of all molecular sciences, which describes matter by the action, interaction and motion of the elementary particles. This science is of vital importance to physicists, chemists, material scientists, and biologists with a molecular view of the world. A full relativistic treatment of atoms and molecules which includes the quantization of the electromagnetic field is currently one of the most challenging tasks in electronic structure theory. Therefore, relativistic effects in atoms and molecules were studied computationally. A combination of wave function and density functional based methods within a correct relativistic framework proved necessary to achieve accurate results of various atomic and molecular properties.