The overall objective of this study was to examine the links between teacher-student relationship quality and student engagement, delimited to affective and behavioural engagement. We used a sequential explanatory mixed methods research approach that consisted of a quantitative phase, in which survey data were collected and analysed within a short-term longitudinal design, followed by a qualitative phase, in which focus group interviews and constructed grounded theory analysis were conducted. Participants included 234 students from two Swedish compulsory schools in the quantitative phase, and 120 in the qualitative phase. The quantitative findings revealed that teacher-student relationship quality predicted student engagement one year later, even when controlling for sex, age, and prior student engagement. The longitudinal association between teacher-student relationship quality and student engagement was unidirectional. The qualitative findings reported students' own perspectives on what they considered to be a good teacher and their ideas of how their teachers and classroom setting influence their affective and behavioural engagement at school. Two significant categories emerged: 'teacher being' and 'teacher doing'.