A Professional Employer Organization (PEO) is a company that establishes a contractual arrangement with a client company to share responsibility and liabilities for the business of employment.

In this contractual relationship among a PEO, a client company, and work site employees, there exists a co-employment relationship in which both the PEO and the client company have an employment relationship with the employee. The PEO is the "administrative employer", while the client company is the "job-site employer". In this arrangement, the PEO and the client company allocate some responsibilities and liabilities, and share others.