Teaching is a complex undertaking and often requires managing unforeseen circumstances, unexpected situations and difficult dilemmas. This paper reports on an empirical study of 22 primary science student teachers planning, conducting and evaluating a science lesson with students aged 4-11. The aim of the study was to empirically illustrate how student teachers through reflecting on incidents within the process of planning, conducting and evaluating their first primary science lesson, perceive teaching concerns relevant for their future practice. The critical incidents were identified as situations within the teaching context that put the student teachers on the spot leading to a revision of their immediate feelings, their planned teaching and of their understandings of science.