Teacher-pupil relationships do not solely impact children's academic development; they also influence emotional and behavioural development. Positive teacher-pupil relationships help reduce maladaptive behaviour while negative ones can lead to increased academic, social and behavioural difficulties. Identifying and measuring teacher-pupil relationship through classroom behaviours and interactions is therefore important as an index of classroom-based influences on child outcome. This article illustrates the development and testing of a classroom observation measure--the Teacher-Pupil Observation Tool (T-POT)--in a small-scale study of 12 teachers and 107 children in 12 reception classrooms. To assess validity and reliability, observed behaviours on the T-POT were correlated with each other (internal validity), and with teacher rated reports of classroom behaviour (discriminant and concurrent validity). Observer agreement was also assessed (inter-rater reliability). The T-POT showed promising psychometric properties in all cases. The T-POT is currently the main outcome measure in four funded evaluation studies in Gwynedd, North Wales, and an outcome measure in a funded evaluation study in Clondalkin, Ireland. These studies will further benefit the measure's validity and reliability and test the sensitivity of the T-POT in detecting change in teacher-pupil behaviour over time. (Contains 1 note and 4 tables.)