Reasoning is one of the proficiency strands of the new Australian Curriculum. It has always been important in mathematics and its importance has always been recognised in mathematics curricula across Australia. However, the new proficiency strand provides an opportunity for all teachers to reconsider how they teach this essential aspect of mathematics. There are many aspects to reasoning in mathematics, but this focuses on the reasoning that establishes why mathematical results are true. Mathematics is distinguished amongst the areas of human knowledge by the special way in which claims of what is true are justified. The assumptions (technically called axioms) and definitions are stated, and gradually, piece by piece, all other mathematical knowledge is built up using the rules for logical deduction. It is an enormously complex web, but the consequence is that mathematical results can be definitely proved. This is not true to nearly the same extent for any other subject. In this article, the author discusses how this fundamental characteristic of mathematics can be conveyed at school. (Contains 2 figures.)