The evolution of density fluctuations in the early universe inevitably generates gravitational waves. Cosmic microwave background observations measured such primordial density fluctuations on the largest scales in our universe (> Mpc). These large-scale fluctuations began their evolution days or years after the Big Bang. How was the universe at earlier times and on smaller scales? At the moment, we do not know much. But gravitational wave observations will provide new information in the next decades. In this talk, I will provide an overview of the so-called induced gravitational waves and how they can be used to probe the presence of primordial black holes. I will place emphasis on the effect of particle interactions on the resulting gravitational wave spectrum. On the occasion of the MITP conference, I will briefly discuss the implications of detecting high-frequency GWs for the primordial black hole scenario.