After a decade of intense studies using the latest X-ray and optical telescopes, the nature of ultraluminous X-ray sources (ULXs) is still largely unknown. No definitive answer has emerged on the question of the mass of the black hole powering these objects (intermediate-mass or supercritical stellar-mass?). Further, we lack even basic knowledge about the binary systems and the companion stars. We will review the properties of the optical counterparts of these systems, from where most properties can be derived. Among the burning issues are: what is the mass donor in these systems? Are they giant/supergiant stars or main-sequence stars? What is the donor mass? Is the optical light from the companion stars or dominated by the accretion disk? We will present new results from recent HST, Chandra, and VLT observations of ULXs addressing these questions.