Discusses the definition of frequency-dependent electric and magnetic multipole moments. It is shown from classical electrodynamics that there are two types of magnetic multipole moments M_E(lm omega ) and M_M(lm omega ), defined as moments of the magnetisation with an electric or magnetic spherical vector wave as weight function. The moment M_E(lm omega ) contributes to the emission of electric multipole radiation. The authors define quantum-mechanical multipole operators in correspondence with the classical case and discuss Siegert's theorem (1937).