Cognitive load theory (CLT) can explain the challenges faced by student teachers. This study, guided by the CLT, included 26 pre-service teachers. Participants completed a cognitive load self-report questionnaire and were interviewed at two points during their student teaching. Results revealed that student teachers decreased mental effort related to monitoring their students' level of attention, meeting needs of diverse learners, and managing internal and external distractions. Qualitative analysis revealed: (1) student teachers became aware of limited cognitive resources; (2) lesson planning imposes cognitive load during student teaching; and (3) cognitive overload limits the ability to make modifications during teaching.