Workplace drug and alcohol testing is used by employers to screen applicants and test employees for illegal or unauthorized drug and alcohol use. For many U.S. employers, drug and alcohol testing has become standard practice, whether or not required by law. An effective and lawful testing program contains a number of important components that employers should consider when beginning a program, assessing a current policy or retooling an existing program to meet changing needs. This article discusses the background of workplace testing; the reasons employers conduct testing; and the various constitutional, statutory and regulatory requirements for public and private employers in certain industries and locations. It provides HR professionals with the basic framework for administering a program and managing test results in a nondiscriminatory and legally compliant manner. It concludes with a discussion of the importance of program monitoring and assessment of benefits for measuring the return on investment.