Abstract Ultraluminous X-ray sources (ULXs) are likely to include different physical types of objects. We discuss some possible subclasses, reviewing the properties of a sample of ULXs recently observed by Chandra and XMM-Newton. Sources with an isotropic X-ray luminosity up to a few times 1039 erg s-1 are consistent with "normal" stellar-mass X-ray binaries (mostly high-mass X-ray binaries in star-forming regions). Higher black hole (BH) masses (≈ 50–100M☉) may be the end product of massive stellar evolution in peculiar environments: they may explain ULXs with luminosities ≈ 1–2 × 1040 erg s-1. Only a handful of ULXs require a true intermediate-mass BH (M 500M☉). Finally, a small subclass of ULXs shows flaring or rapid variability in its power-law spectral component.
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