Extra-solar X-ray astronomy began with the historical paper in Physical Review Letters by Giacconi, Gursky, Paolini, and Rossi (1962). Now, more than four decades later, X-ray astronomy is central to many aspects of astronomy. In 2002, Riccardo Giacconi was awarded the Nobel Prize in Physics “for pioneering contributions to astrophysics, which have led to the discovery of cosmic X-ray sources”. In the decade since the publication of X-ray Binaries – the predecessor of the present book – the study of compact stellar X-ray sources has received enormous impetus from observations with the BeppoSAX, Rossi X-ray Timing Explorer (RXTE), Chandra, and XMM-Newton X-ray observatories. In addition, many exciting new results on these X-ray sources have also been produced in the radio, infrared, optical and ultraviolet bands. Highlights include the discovery in low-mass X-ray binaries of millisecond X-ray pulsations, confirming the connection with the millisecond radio pulsars. Millisecond and sub-millisecond quasi-periodic oscillations (QPO) were discovered that are thought to provide a direct view of regions of strong-field gravity near neutron stars and black holes. The discovery of X-ray, optical and radio afterglows of gamma-ray bursts (GRB) firmly established their long-suspected cosmological distances. Super-luminal motion of radio jets was discovered in accreting black-hole binaries. Dozens of ultra-luminous X-ray sources (ULX) have been detected in many galaxies. Their origin is still not clear; some may be accreting intermediate-mass (i.e., of order 103 M⊙) black holes (IMBH).