Scaling up instructional improvement remains a central challenge for school systems. While existing research suggests that teachers' social networks play a crucial role, we know little about what dimensions of teachers' social networks matter for sustainability. Drawing from a longitudinal study of the scale-up of mathematics reform, we use qualitative social network analysis and qualitative comparative analysis (QCA) to investigate the relationship between teachers' social networks and sustainability. Teachers' social networks in the first 2 years of the initiative influenced their ability to sustain reform-related instructional approaches after supports for reform were withdrawn. Social networks with combinations of strong ties, high-depth interaction, and high expertise enabled teachers to adjust instruction to new conditions while maintaining the core pedagogical approach. This research contributes to our understanding of the dynamics of sustainability and to social network theory and research. (Contains 5 tables, 1 figure and 6 notes.)