This study aims to reveal the causes and consequences of school principals' favouritism behaviours based on teachers' perceptions, and definitions about favouritism behaviour within schools. A descriptive phenomenological design was applied. Data were gathered via face-to-face interviews conducted with 15 public middle school teachers in Turkey. The data was then examined through content analysis. The participants defined the characteristics of favouritism behaviour as injustice, inequality, ignoring, partiality, and privilege, with types of favouritism classified as indicators of distribution, administrative process, and communication-interaction. The causes of favouritism behaviours were stated as a desire to protect the individuals they are with, to increase the power, to gain interest, and to cover their own incompetence. It was determined that favouritism behaviours have negative effects on both an individual and institutional level. Based on the findings, it was suggested to develop specific management strategies in order to prevent negative consequences of favouritism behaviours.