Our study examines youth attitudes towards unions over a 40-year period to try and understand whether today's young workers might be the 'hero' or the 'villain' in the tale of declining union membership rates in the United States. Using nationally representative time-lag data from high-school seniors (N = 104,742) spanning 1976-2015, we conducted time trend, birth cohort and generational analyses to provide an 'apples to apples' comparison of how youth have felt about unions at different points in time. We found that contemporary youth (or Millennials) hold similar union attitudes to those who came before them, though what predicts those attitudes has changed over time. Strikingly, we also found that the proportion of young people who hold no opinion about unions has more than doubled over the period under study, steadily rising from 14 per cent in 1976 to 33 per cent in 2015. This sizeable proportion of 'agnostic' youth should be alarming to unions, yet it also provides them with opportunities to shape youth attitudes through targeted outreach efforts.