The Service Employees International Union (SEIU) is one of the largest labor unions in North America, representing approximately 2 million service workers across more than 100 occupations in the United States, Canada, and Puerto Rico. Founded on April 23, 1921, in Chicago as the Building Service Employees International Union (BSEIU) by immigrant janitors, it initially focused on building service workers. Through strikes, mergers, and aggressive organizing, it grew significantly, renaming itself SEIU in 1968 to reflect expansion into healthcare, public services, and property services. Today, it continues to advocate for better wages, healthcare, and worker rights through organizing, collective bargaining, strikes, and political action, representing workers in hospitals, home care, nursing homes, public sector jobs, and property maintenance. SEIU has played a pivotal role in advancing rights for women, immigrants, and people of color, disaffiliated from the AFL-CIO in 2005 to form the Change to Win Federation, and remains a political and bargaining powerhouse despite facing internal and external challenges.