This paper looks at research based models that have been put forward to describe how teacher use of technology develops. There have been a variety of studies that have proposed levels that describe teacher use of technology and identify the different aspects of use that distinguish these levels. Very few of these studies have shown how teachers progress from one level to another. In this paper we examine data collected from a small group of teachers who completed logbooks and undertook interviews regarding their use of graphics calculators. This data has been used alongside the existing literature to develop a hierarchical model of teacher development, which tries to take account of the factors that promote movement from one level to another. (Contains 5 tables and 1 figure.)