Relativistic effects on molecular properties and energies are ubiquitous in chemistry. Their consideration in quantum chemical calculations requires Dirac's theory of the electron, whose application is not without obstacles. Douglas–Kroll–Hess theory accomplishes a decoupling of positive- and negative-energy eigenstates of the Dirac one-electron Hamiltonian by an expansion in the external potential. At low orders, this expansion already converges and provides efficient relativistic Hamiltonians to be used in routine quantum chemical calculations. The basic principles of the approach are reviewed, and most recent developments are discussed. © 2011 John Wiley & Sons, Ltd.