Abstract: Supermassive black holes (BHs) live at the heart of essentially all massive galaxies with bulges, power AGN, and are thought to be important agents in the evolution of their hosts. However, the origin of the first supermassive BH "seeds" is far from understood. While direct observations of these distant BHs in the infant Universe are unobtainable with current capabilities, massive BHs in present-day dwarf galaxies offer another avenue to observationally constrain the masses, host galaxies and formation path of supermassive BH seeds. Using optical spectroscopy from the SDSS, we have increased the number of known dwarf galaxies hosting massive BHs by more than an order of magnitude. These dwarf galaxies have stellar masses comparable to the Magellanic Clouds and contain some of the least-massive supermassive BHs known. I will present results from this study, and well as on-going efforts using radio and X-ray observations to reveal massive BHs in star-forming dwarfs that can be missed by optical diagnostics.