We review the state of the art in work site health promotion (WHP), focusing on factors that influence the health and productivity of workers. We begin by defining WHP, then review the literature that addresses the business rationale for it, as well as the objections and barriers that may prevent sufficient investment in WHP. Despite methodological limitations in many available studies, the results in the literature suggest that, when properly designed, WHP can increase employees' health and productivity. We describe the characteristics of effective programs including their ability to assess the need for services, attract participants, use behavioral theory as a foundation, incorporate multiple ways to reach people, and make efforts to measure program impact. Promising practices are noted including senior management support for and participation in these programs. A very important challenge is widespread dissemination of information regarding success factors because only approximately 7% of employers use all the program components required for successful interventions. The need for more and better science when evaluating program outcomes is highlighted. Federal initiatives that support cost-benefit or cost-effectiveness analyses are stressed, as is the need to invest in healthy work environments, to complement individual based interventions.