Abstract: Ultraluminous X-ray sources (ULXs) are a special class of celestial objects found in nearby galaxies with X-ray emission. They behave like the black hole binaries in our Milky Way but have a higher X-ray luminosity, indicating that they may harbor more massive black holes which are the so called intermediate-mass black holes (IMBHs). IMBHs are interesting in astrophysics because they cannot be formed in the core collapse of a massive star at the end point of its evolution, which is speculated to be the mechanism how a stellar-mass black hole is formed. This article describes basic characteristics of ULXs, reviews important multiwavelength observations made in recent years of these sources, and discusses how the observations would place constraints on the nature of ULXs.