This study aims to examine how teachers’ mathematical and pedagogical knowledge develop as they learn to use a multi-representational technological tool, the TI-Nspire handheld device and computer software. It is conducted as an enquiry into the learning trajectories of a group of secondary mathematics teachers as they begin to use the device with a focus on their interpretations of mathematical variance and invariance. The research is situated within an English secondary school setting and it seeks to reveal how teachers’ ideas shape, and are shaped by, their use of the technology through a scrutiny of the lesson artefacts, semi-structured interviews and lesson observations. Analysis of the data reveals the importance of the idea of the ‘hiccup’; that is the perturbation experienced by teachers during lessons stimulated by their use of the technology, which illuminates discontinuities within teachers’ knowledge. The study concludes that the use of such a multi-representational tool can substantially change the way in which both the teachers and their students perceive the notions of variance and invariance within dynamic mathematical environments. Furthermore, the study classifies the types of perturbations that underpin this conclusion. The study also contributes to the discourse on the design of mathematical problems and their associated instrumentation schemes in which linked multiple representations offer a new environment for developing mathematical meanings. This thesis makes an original contribution to understanding what and how teachers learn about the concept of mathematical variance and invariance within a technological environment.