We review the role which classical trajectories play in quantum-mechan
ical systems in a more or less chronological order guided by experimen
tal observations. The onset of a renewed interest in classical dynamic
s was catalysed by the observation of unknown features in the absorpti
on spectra of atoms in the presence of external magnetic and electric
fields. Although the most dominant features in these spectra can be ac
counted for by the simple quantisation conditions for classical electr
ons, the finer details require a more sophisticated approach. Starting
from the Feynman path-integral formalism, Gutzwiller's periodic-orbit
theory is introduced, which is then transformed into the closed-orbit
theory developed by Delos et al. Applying this semiclassical theory e
nabled atomic physicists to understand complicated quantum spectra as
a coherent sum of classical trajectories. Newly observed features such
as core-scattered orbits and ghost orbits, necessitated adjustments t
o the standard closed-orbit theory in order to be able to reproduce th
ese new features. Recent results from both scaled-energy experiments a
nd wave-packet experiments are presented to demonstrate the classical
dynamics underlying the quantum system as it is currently understood.