Purpose: The purpose of this paper is to assess leadership learning in a principalship development programme. Design/methodology/approach: This case study adopted Popper's three worlds as an analytical framework to assess leadership learning in a principalship development programme. The unit of assessment of learning is knowledge--more specifically subjective and objective knowledge. The study involved participants in a principalship development programme called the Leaders in Education Programme (LEP). The LEP is designed and implemented by the National Institute of Education, Singapore with partnership from the Ministry of Education, Singapore. Since its inception in 2001, the LEP has developed more than 300 school leaders in Singapore schools. Findings: Through the analyses linking the concepts of complexity theory and Popper's three worlds, valuable insights were gained into how participants developed practical knowledge in leadership and management. Originality/value: The outcome of the analyses has implications for leadership programme design and for leadership development in countries with similar challenges to prepare a new type of school leaders for the 21st century. (Contains 2 figures and 1 table.)