X-ray observations of classical and recurrent novae at different stages aft
er the outburst are reviewed in this paper. I introduce the three mechanism
s of X-ray emission, present some old and recent observations done with ROS
AT and SAX, and discuss the results of a survey of ROSAT serendipitous obse
rvations of 50 novae. (C) 1999 Elsevier Science B.V. All rights reserved.