Plumbers install pipes for water, gas, and medical gas systems. They also install sanitary facilities in residences, commercial, and industrial settings. This includes multi-housing complexes, retail, medical and research centers, and nearly any commercial structure requiring sanitary systems. Plumbers use a variety of academic and technical skills to install, repair, and maintain systems. Plumbers run tests on their installations to ensure the system functions properly and meets the plumbing code. Plumbing is a licensed craft, and professionals must maintain their license and specialty credentials throughout their careers.