This study documented the design, development and refinement of a professional development program, which improved teacher educators' knowledge, to facilitate their effective use of technology in teaching mathematics. The study was conducted in two Ethiopian Colleges of Teacher Education. It involved 16 mathematics teacher educators, four Information Communication Technology coordinators, and 247 elementary pre-service mathematics teachers. The study was undertaken across three phases. The first phase comprised a contextual and problem analysis and development of a conceptual framework based on a literature review that provided the basis to design the first professional development program prototype. The second phase involved setting out design guidelines and optimising the professional development program prototype through two cycles of design, formative refinement and revision. The last phase of the study was an evaluation of the extent to which the professional development program that took place over 5 months had met its objectives. The study used a mixed methods approach, which involved the collection of both quantitative and qualitative data informed by an educational design research methodology. Data were collected using questionnaires, observation checklists, semi-structured interviews, focus group discussions and professional learning workshops. One of the findings of the study in the first phase showed that the teacher educators believed they had little knowledge of technology and its effective use in their teaching of mathematics, and that this was a major barrier resulting in limited technology use in their teaching. The first phase of the study informed the design of the professional development program in the second phase of the study. The designed professional development program was characterised by its focus on the pedagogical use of technology, provision of exemplar material, the use of available and webbased technologies, the formation of teams, support from Information Communication Technology (ICT) coordinators, and an emphasis on informality in the professional development arrangements. The results in the third phase of the study showed that the professional development program contributed to increased use of technology by the mathematics teacher educators and their pre-service teachers. The teacher educators' perceived knowledge of how to use and integrate technology teaching practices in their teaching of mathematics was improved and this was associated with positive changes in their effective technology integrated mathematics teaching as evidenced by the classroom observations.