  Since the launch of Chandra and XMM-Newton, high-resolution X-ray spectra of
cosmic sources of all kinds have become available. These spectra have resulted
in major scientific breakthroughs. However, due to the techniques used, in
general high-quality spectra can only be obtained for the brightest few sources
of each class. Moreover, except for the most compact extended sources, like
cool core clusters, grating spectra are limited to point sources. Hitomi made
another major step forward, in yielding for the first time a high-quality
spectrum of an extended source, and improved spectral sensitivity in the Fe-K
band. For point sources with the proposed Arcus mission, and for all sources
with the launch of Athena, X-ray spectroscopy will become mature. It allows us
to extend the investigations from the few handful of brightest sources of each
category to a large number of sources far away in space and time, or to get
high time-resolution, high-spectral resolution spectra of bright time variable
sources.
