This paper deals with a crucial topic: which factors influence the sustainability and scale-up of a professional development programme's impact? Theoretical models and empirical findings from impact research (e.g. Zehetmeier and Krainer, "ZDM Int J Math" 43(6/7):875-887, 2011) and innovation research (e.g. Cobb and Smith, "International handbook of mathematics teacher education," vol 3, pp 231-254, Sense Publishers, Rotterdam, 2008; Rogers, "Diffusion of innovations," Free Press, New York, 2003) are combined in order to be used as a theoretical framework for a qualitative impact analysis of a mathematics teacher professional development programme in Austria. The paper provides data from document analyses and interview series to describe as well as explain this programme's various impacts on different levels within a case study setting. In particular, this study focuses on factors (e.g. networks, shared vision or mutual accountability) which are influencing the scale-up and sustainability of a professional development programme's impact (e.g. on teachers' knowledge, beliefs or practice). Finally, implications for upcoming professional development programmes are discussed.