To systematically document key characteristics and features of American Job Centers (AJCs), Mathematica and its partners—Social Policy Research Associates, The George Washington University, and Capital Research Corporation—conducted the Institutional Analysis of AJCs for the U.S. Department of Labor.  This paper describes the role and activities of One-Stop Operators in the AJC system based on site visits to 40 comprehensive American Job Centers (AJCs). It provides an overview of the types of entities that served as Operators, the roles that Operators played, common supervision models, and the key activities of AJC managers in day-to-day center operations. It concludes with a description of Workforce Innovation and Opportunity Act (WIOA) One-Stop Operator requirements and identifies some general issues and challenges identified by local areas as they prepared to implement the new One-Stop Operator requirements.