This chapter introduces the notion that current definitions of instructional leadership, while generally useful, need to be reconceptualised to acknowledge the power of assessment in its many forms. In this chapter a discussion of various definitions of instructional leadership is examined to identify common dimensions of this leadership orientation. The argument is further developed in describing the changes in thinking related to instructional design and the pivotal role assessment should now play in designing appropriate and educational learning experiences. Assessment and inclusion are examined as another key dimension, and one that is frequently overlooked or addressed as a separate issue for leaders, rather than an embedded component of instructional leadership. The final dimension offered as part of this new conceptualisation is to examine how assessment influences the motivation of students and their teachers and is used within socio-political debates. Overall these dimensions are discussed in order to highlight their importance and the need for leaders to acknowledge the power of assessment within their instructional leadership responsibilities.