Drawing on research into students' understanding, this article argues that the customary approach to introductory chemistry has created difficulties for students. Instead of being based on the notion of "solids, liquids and gases", introductory chemistry should be structured to develop the concept of a substance. The concept of a substance is defined and how the absence of this concept causes difficulties is explained. Important developments in the new National Curriculum in England are highlighted and taken further to show a curriculum structured around the concept of a substance.