Almost all four-year institutions of higher education have adopted the tenure system as a formal policy for faculty employment. The degree to which tenure systems are actually implemented, however, depends on resource flows and institutional pressures. Fewer resource constraints (i.e., greater per-student revenues and larger endowments) increase the proportion of professors employed on tenure-track lines; likewise, a stronger research orientation positively affects the share of faculty in tenure-track slots. Colleges and universities that rely more heavily on tuition for revenues and those with larger numbers of accreditations (from professional and occupational associations) generally employ fewer tenure-track professors. Other variables also matter: Tenure is more prevalent at public, older, and more complex universities and colleges and is less widespread among institutions that enroll larger numbers of students and among those that include a medical school. And finally, the share of tenure-track faculty declines on campuses with a larger pool of graduate students who are available to teach.