Classical physics is physics based on principles developed before the rise of quantum theory. It includes special theory of relativity as well. There are no restrictions on application of classical principles, but, practically, the scale of classical physics is the level of isolated atoms and molecules on upwards, including the macroscopic and astronomical realm. Inside the atom and among atoms in a molecule, the laws of classical physics break down and generally do not provide a correct description. Moreover, the classical theory of electromagnetic radiation is somewhat limited in its ability to provide correct descriptions, since light is inherently a quantum phenomenon. Unlike quantum physics, classical physics is generally characterized by the principle of complete determinism.

A physical system on the classical level is a physical system in which the laws of classical physics are valid.

Among the branches of theory included in classical physics are:

• Classical mechanics
  • Newton's laws of motion
  • Classical Lagrangian and Hamiltonian formalisms
• Classical electrodynamics (Maxwell's Equations)
• Classical thermodynamics
• Special theory of relativity and General theory of relativity
• Classical chaos theory and nonlinear dynamics

Mathematically, classical physics equations are generally ones in which Planck's constant do not appear.

The current paradigm of physics is that the fundamental laws of nature are the laws of quantum physics, and that classical theory is the by-product of quantum rules applies to the macroscopic realm. At present, this conjecture is more easily assumed than proven, however, and one of the most vigorous on-going fields of research in physics is classical-quantum correspondence . This field of research is concerned with the discovery of how the laws of quantum physics give rise to classical physics in the limit of the large scales of the classical level.