After presenting the pros and cons of instituting worker seniority systems, the author reviews various definitions of seniority. Descriptions of the principal applications of seniority focus on job security, benefit qualification, benefit amount, and seniority as a mitigating factor in discipline cases. An analysis of seniority issues in job-security arbitration cases addresses seniority as a criterion in decisions involving job security, variations of seniority usages in job-security cases, and special problems involving seniority and layoffs. A review of potential conflicts between seniority systems and principles of equal employment opportunity discusses the impact of Title VII of the 1964 Civil Rights Act and the arbitrator's role in deciding cases that involve legal issues. The paper also considers the following miscellaneous issues involving the application of seniority in collective bargaining: filling job vacancies, granting greater seniority to union stewards and officers; and the continuation, interruption, or termination of seniority. The paper's concluding section presents a checklist of the issues that should be considered by parties negotiating a seniority provision in a labor contract.