This chapter outlines Piaget’s Stage Theory of Cognitive Development and its application in a teacher training institute involving graduate physics trainee teachers predicted to operate at the formal operational stage. It is presumed that, at this stage, having assimilated and accommodated logical understanding from the concrete operational stage as well as having developed the ability to think critically from the concrete operational and formal operational stages, the trainees are hypothesized to have a good articulation of theoretical, combinatorial and proportional reasoning. In this study, an attempt has been made to investigate this hypothesis through a case study involving six physics trainee teachers. The trainees were assigned a written task on the motion of a golf ball after which they were required to verbalise their reasoning patterns during individual oral presentations. The trainees demonstrated adequate theoretical reasoning but lacked combinatorial and proportional reasoning patterns. We opine that teacher training institutions should design programmes that make provision for the interconnection of concrete and formal operational stages into a cyclical mode so as to promote critical thinking and reflective practices in trainees.