Ultraluminous X-ray sources (ULXs) are extragalactic objects that are located outside the nuclei of their host galaxies with luminosities that exceed the Eddington limit for a stellar mass black hole. These luminosities have been interpreted as evidence of a new class of intermediate mass black holes with masses between ~100 - 100,000 Msun. The brightest of these objects (the hyperluminous X-ray sources) have luminosities above 1E41 erg/s that cannot be easily explained without intermediate mass black holes. However, the luminosities of the bulk of ULXs could be explained through a combination of mild super-Eddington accretion and beaming. Here I will describe the multi-wavelength observing campaign targeting ESO 243-49 HLX-1, the most luminous ULX currently known and the strongest candidate intermediate mass black hole. I will also present the results of a recent study into transient behaviour in ULXs that indicates that the bulk of the population may not be able to be explained via hyper-accretion onto stellar mass black holes with masses between ~10 - 30 Msun.