  The 21st century X-ray observatories XMM-Newton, Chandra, and Swift gave us
completely new insights into the X-ray behaviour of nova outbursts. These
new-generation X-ray observatories provide particularly high spectral
resolution and high density in monitoring campaigns, simultaneously in X-rays
and UV/optical. The entire evolution of several nova outbursts has been
observed with the Swift XRT and UVOT instruments, allowing studies of the
gradual shift of the peak of the SED from UV to X-rays, time scales to the
onset and duration of the X-ray brightest supersoft source (SSS) phase, and
pre- and post-SSS X-ray emission. In addition, XMM-Newton and Chandra
observations can efficiently be scheduled, allowing deeper studies of
strategically chosen evolutionary stages. Before Swift joined in 2005, Chandra
and XMM-Newton blind shots in search of SSS emission unavoidably led to some
underexposed observations taken before and/or after the SSS phase. More
systematic Swift studies reduced this number while increasing the number of
novae. Pre- and post-SSS spectra at low and high spectral resolution were
successfully modelled with collisional plasma models. Pre-SSS emission arises
in shocks and post-SSS emission in radiatively cooling thin ejecta. In
contrast, the grating spectra taken during the SSS phase are a lot more complex
than expected and have not yet been successfully modeled. Available hot WD
radiation transport models give only approximate reproduction of the
observations, and make some critical assumptions that are only valid in
isolated WDs. More grating spectra which would be important to search for
systematic trends between SSS spectra and system parameters.
