Since undocumented workers are a serious issue in the US, the USCIS makes it a requirement that the work authoritarian of every employee be evaluated — even the authorization of employees who are citizens or green card holders. USCIS Form I-9 (Employment Eligibility Verification) allows employers to evaluate each employee for work eligibility in the US. By law, all employers are required to complete and file a Form I-9 for every employee hired. To fill out the form, employers must ask employees to see their authorization of employment; acceptable proof of authorization is listed on the form for employers to refer to.