As of 11 July 2012, we know of five moons around Pluto.  The largest, Charon, was discovered in June 1978, and is closer to its parent in size than any other moon in the Solar System.  In fact, when it was first discovered it was found that the combined mass of the two had thrown out all calculations of Pluto’s size.  Up till then, Pluto was assumed to be much larger than we know it is today.  The next two moons, Hydra and Nix, were discovered by the Hubble Space Telescope in 2005 (although technically the hubble photographed them in 2002 — it took three years for anybody to notice the tiny dots on the picture that turned out to be moons!).  The fourth, sized somewhere between 13 and 34 kilometers across, is called Kerberos.  The fifth, and smallest (probably somewhere between 10 and 25km) was the most recently discovered and is known as Styx.