Recent declines in the average length of time that U.S. workers spend with a given employer represent an important change in the nature of the employment relationship, yet it is one whose causes are poorly understood. I explore those causes using Current Population Survey data on the tenure of men aged 30–65, from the years 1979–2008. I argue that long-term employment relationships primarily occur when workers pressure employers to close off employment from market competition, reducing the attractiveness of external mobility relative to internal opportunities and increasing employment security. I then explore how two changes in organizations’ environments—a decline in union strength and increased turbulence from changes in technology and globalization—might have affected workers’ ability to secure such closed employment relationships over the last 30 years. My results support the argument that declines in tenure reflect the reduced power of workers to secure closed employment relationships. Recent declines in tenure have been concentrated in large organizations, and many of those declines are explained by controlling for the changing levels of industry unionization. I find little evidence that foreign competition or technological change affected mobility. The results are robust to measures of changing industry growth rates and within-industry reorganization. Supplementary analyses suggest that layoffs are associated with different industry pressures than tenure and that voluntary mobility may have played an important role in declines in tenure.