  We explore the nature of ultraluminous X-ray sources through detailed
investigations of their spectral shape using some of the highest quality data
available in the XMM-Newton public archives. Phenomenological models allow us
to characterise their spectra, while more 'physically-motivated' models enable
us to explore the physical processes underlying these characteristics. These
physical models imply the presence of extreme (probably super-Eddington)
accretion on to stellar mass black holes.
