  The intracluster medium (ICM) contains the vast majority of the baryonic
matter in galaxy clusters and is heated to X-ray radiating temperatures. X-ray
spectroscopy is therefore a key to understand both the morphology and the
dynamics of galaxy clusters. Here we recall crucial evolutionary problems of
galaxy clusters unveiled by 19 years of high-resolution X-ray spectroscopy with
the Reflection Grating Spectrometer (RGS) on board XMM-Newton. Its exquisite
combination of effective area, spectral resolution and excellent performance
over two decades enabled transformational science and important discoveries
such as the lack of strong cooling flows, the constraints on ICM turbulence and
cooling-heating balance. The ability of RGS to resolve individual ICM spectral
lines reveals in great detail the chemical enrichment in clusters by supernovae
and AGB stars. RGS spectra clearly showed that the ICM plasma is overall in
thermal equilibrium which is unexpected given the wealth of energetic phenomena
such as jets from supermassive black holes and mergers.
