Structural thinking skills should be developed as a prerequisite for a young person's future mathematical understanding and a teachers' understanding of mathematical structure is necessary to develop students' structural thinking skills. In this study, three secondary mathematics pre-service teachers (PSTs) learned to notice structural thinking through the CRIG framework of mathematical structure. The framework consists of Connections, Recognising patterns, Identifying similarities and difference, and Generalising and reasoning. I report here on how the CRIG framework helped the PSTs' notice structural thinking.