Integrating technology in education is still not an easy task. Teachers' adoption of technology in their teaching is even more problematic. This study aims at understanding what critical factors affect technology adoption by in-service mathematics teachers. This article is one out of four parts of the study. The type of the study is a multiple case studying in depth the effect of a GeoGebra (a free mathematics software) intervention on the Technological Pedagogical Content Knowledge (TPACK) of in-service mathematics teachers in secondary schools who follow the Lebanese curriculum. The methodology used is Design-Based Research that focuses on working closely with practitioners in collaborative and iterative manner in the real context to add principles to theory and practice. Results showed an increase in the level of TPACK domains of teachers especially in their student-centered teaching approach.