The issue of sustaining and scaling up professional development for mathematics teachers raises several fundamental issues for researchers. This commentary addresses various definitions for sustainability and scaling up and how these definitions may affect the design of programs as well as the design of research. We consider four of the papers in this issue. Two of the papers describe programs which were designed from the beginning, based on theoretical considerations, to support the possibility of long-term sustainability and scaling up the program. These papers assess the results of those specific aspects of the program which aimed to influence sustainability and scaling up. The other two papers are concerned with viewing programs from a distance and assessing which factors contributed to or hindered sustaining and scaling up the programs. Based on the four papers, factors which promote or impede sustainment and scaling up are examined; these factors include motivation, content, and processes. For example, all four studies incorporated the element of collectivity, some as content, some as processes, and some as both. We address the question of how important this element is for sustainability and scaling up. Factors beyond the program such as the school environment are also reviewed. We question the degree of support necessary for sustainment and scaling up, or even the necessity for system support, and suggest that the influence of this factor may depend on the particular aspect of professional development a teacher chooses to sustain. The inter-relationship between sustainability and scaling up is discussed throughout. We propose a view of sustainability that takes into account a dynamic environment, where an impact is sustained if it continues to flourish, and scaling up can take place if the desired outcome is sustainable.