There is a growing body of literature exploring pedagogies that promote children's happiness at school. Yet there is little research on teacher's conceptions of happiness, and how this informs their decisions about children. This paper reports on findings from a qualitative study exploring teachers' conceptions of happiness. Twelve teachers from Waldorf, Froebel, Montessori, public, and independent schools were interviewed, followed by ethnographic observations in eight Kindergarten through Grade 2 classrooms. Analysis identified three themes: landmarks of happiness, experiences of happiness and unhappiness, and barriers and tensions. This paper concludes with a discussion on the need for school leadership to promote mental health for both children and teachers, offers practical suggestions for developing happy classrooms, and outlines implications for further research.