  With exactly forty years since the discovery of the first cyclotron line in
Her X-1, there have been remarkable advancements in the field related to study
of the physics of accreting neutron stars -- cyclotron lines have been a major
torchbearer in this regard, from being the only direct estimator of the
magnetic field strength, a tracer of accretion geometry and an indicator of the
emission beam in these systems. The main flurry of activities have centred
around studying the harmonic separations, luminosity dependence, pulse phase
dependence and more recently the shapes of the line and the trend for long term
evolution in the line energy. This article visits the important results related
to cyclotron lines since its discovery and reviews their significance. An
emphasis is laid on pulse phase resolved spectroscopy and the important clues a
joint timing and spectral study in this context can provide, to build a
complete picture for the physics of accretion and hence X-ray emission in
accreting neutron stars.
