This paper examines the relationship between job seniority and wages, a key aspect of specific human capital theory and other compensation theories. The author uses longitudinal data to estimate the average return to job seniority among adult men, finding that 10 years of current job seniority increases the wage of the typical male worker by over 25%. This suggests that accumulation of specific capital is crucial in employment relationships and life-cycle earnings. The paper also discusses the potential biases in survey data due to job-changing decisions and provides a two-stage estimation procedure to address these biases. The results indicate that the returns to seniority are substantial and consistent across different occupational categories. The findings challenge previous interpretations that assume minor returns to seniority and highlight the importance of specific capital in labor markets.This paper examines the relationship between job seniority and wages, a key aspect of specific human capital theory and other compensation theories. The author uses longitudinal data to estimate the average return to job seniority among adult men, finding that 10 years of current job seniority increases the wage of the typical male worker by over 25%. This suggests that accumulation of specific capital is crucial in employment relationships and life-cycle earnings. The paper also discusses the potential biases in survey data due to job-changing decisions and provides a two-stage estimation procedure to address these biases. The results indicate that the returns to seniority are substantial and consistent across different occupational categories. The findings challenge previous interpretations that assume minor returns to seniority and highlight the importance of specific capital in labor markets.