School principals' decisions are made within the complex organizations called schools. This study explored how systems thinking is reflected in principals' perceptions about their decision-making processes. Based on a qualitative analysis of interviews and focus groups, principals' descriptions of their own effective decision-making reflected systems thinking in the following three areas: (1) expanding the number of choices; (2) identifying possible consequences of various alternatives; and (3) seeking and analysing relevant information. As school is an inherently complex organization, the findings of this study shed light on the connection between the complicated nature of principals' decision-making and systems thinking. Implications and further research are discussed.