In the 1995 X-ray Binaries book edited by Lewin, van Paradijs and van den Heuvel, the chapter on Normal galaxies and their X-ray binary populations (Fabbiano 1995) began with the claim that "X-ray binaries are an important component of the X-ray emission of galaxies. Therefore the knowledge gathered from the study of Galactic X-ray sources can be used to interpret X-ray observations of external galaxies. Conversely, observations of external galaxies can provide us with uniform samples of X-ray binaries, in a variety of different environments." This statement was based mostly on the Einstein Observatory survey of normal galaxies (e.g., Fabbiano 1989; Fabbiano, Kim & Trinchieri 1992). Those results have been borne out by later work, yet at the time the claim took a certain leap of faith. Now, nearly a decade later, the sensitive sub-arcsecond spectrally-resolved images of galaxies from Chandra (Weisskopf et al. 2000), complemented by the XMM-Newton (Jansen et al. 2001) data for the nearest galaxies (angular resolution of XMM-Newton is ~ 15"), have made strikingly true what was then largely just wishful anticipation.
While a substantial body of ROSAT and ASCA observations exists, which was not included in the 1995 Chapter, the revolutionary quality of the Chandra (and to a more limited degree of XMM-Newton) data is such that the present review will be based on these most recent results.
In this Chapter we first discuss the emerging awareness of X-ray (0.1 - 10 keV band, approximately) stellar populations in spiral galaxies: we focus on four well studied galaxies (M31, M81, M83 and M101), and we then discuss the effect of recent widespread star formation on the luminosity functions of the X-ray emitting populations (Section 2). We then review the body of observational evidence on the ultraluminous X-ray sources (LX > 1039 ergs s-1), that are associated with active/recent star formation (Section 3; see the Chapter by King, in this book, for a review of theoretical work on this subject; see also the chapter by McClintock & Remillard on black hole binaries). We follow with a review of the X-ray population properties of old stellar systems (E and S0 galaxies; Section 4). We then discuss the results of correlation analyses of the integrated galaxy emission (Section 5), and we conclude with a look at the X-ray evolution of galaxies going back into the deep universe (Section 6).