The aim of this paper is to examine the ways students are taught about and learn the particle theory in Grades 6-12. Our own research students conceptions of the particle model (eg. Harrison & Treagust 1996,1999,2000) and our detailed review of the literature lead us to conclude that students' particle knowledge is less than ideal. This view is supported by an Australian survey of 2840 grades 7-12 students (Harrison, Klease and Voss 2005). The survey found that many secondary students still favour continuous models of matter, cannot explain phase changes in terms of particle interactions, have a deficient mental image of the liquid state and are not sure whether steel and light are/are not matter. Only some students can justify the existence of particles. Some classes exhibited sound understanding of the particle concept; however knowledge growth across Grades 8-12 was much smaller than expected and almost zero between grades 9 and 10.