This paper examines various definitions and frameworks that have been used to justify the emergence of a new category for the teacher professional: teacher leader. The emergence of this new professional category may lead to greater retention levels, and improved knowledge management and transfer within the teaching profession. Various key dimensions of this profession are examined, allowing us to highlight some key personal skills that would appear requisite for the teacher leader. An agenda for developing and validating assessments of teacher leadership is then proposed. It is argued that these assessments have the potential of legitimizing research within this field, as well as providing the opportunity to better understand what it takes to become a successful professional in this new domain of teaching practice. Appended are: (1) Kansas Teacher Leader Standards; and (2) What Makes an Assessment "Authentic"? (Contains 6 tables and 1 figure.)