Instructional leadership concerns the ongoing deep involvement of school principals in direct efforts to improve learning and achievement for all students. The goal of this study was to explore principals' perceptions regarding the knowledge required for instructional leadership. Study participants were 38 Israeli school principals, representing the population of Israeli principals in terms of sex, age, seniority, academic degree and school level. Data were collected through semi-structured interviews. Data analysis classified the statements of the study participants according to Shulman's seven categories of knowledge needed for effective teaching. The findings suggested that "General Pedagogical Knowledge" plays a key role in instructional leadership, while somewhat reducing the importance of "Leadership Content Knowledge." Implications and further research of this study are discussed.