Diagrams are succinct, visual representations of real-world or mathematical objects that serve to communicate properties of the objects and facilitate problem solving. This study explored perceptions of mathematics teachers in Singapore, who are heads of mathematics departments in their respective schools, related to diagrams and their use in the teaching of mathematics. An open-ended survey was adopted to illicit responses to (1) what is a diagram, and (2) when do you use diagrams in your mathematics instruction. The findings of the study show that participants generally viewed diagrams as visual representations of real life or mathematical objects and they used them mainly as visual aids when illustrating mathematical concepts and relationships.