Teamwork skills are required at work, but teacher efforts in many countries to track achievement within this context have been hindered by lack of assessment tools and input from students. The Teamwork Skills Inventory relies on peer and self-evaluation to establish accountability, identify competencies, and detect learning needs. Twenty-five items state the criteria students refer to in reporting observations about whether individual members of their cooperative learning group attend to teamwork, seek and share information, communicate with teammates, think critically and creatively, and get along with teammates. Ways to prepare students for authentic assessment and to process anonymous feedback from peers are discussed. A field test of 303 high school students and teachers determined validity and reliability. Students were able to recognize team skills as well as deficits of peers, and gender differences in competencies were acknowledged. A portfolio record of team skills enables teachers across subjects to apply united interventions. (Contains 4 tables.)