HOK (formerly Hellmuth, Obata + Kassabaum) is a global architecture, interiors, engineering, planning and consulting firm. HOK is the largest U.S.-based architecture-engineering firm and the "No. 1 role model for sustainable and high-performance design." HOK also is the second-largest interior design firm. As of 2011, the firm maintains more than 1,800 professional staff across a global network of 25 offices and is active in all major architectural specialties. Its senior leaders are located in several different locations across the world.