This article shows that enabling teachers to integrate comparing solution strategies into their teaching fosters student flexibility in algebra. We designed two professional development (PD) programs that either focused exclusively on comparing solution strategies, or additionally introduced the accountable talk approach to guiding productive classroom discussions. The effects of both PD programs were investigated in an experimental field study (N = 39 teachers, 739 students). In both experimental groups, student posttest gains in strategy flexibility and procedural knowledge were greater than in the control group. The accountable talk group also increased conceptual knowledge. Significant effects in strategy flexibility were still observed 2.5 months later. We discuss recommendations for PD programs to foster flexibility in algebra using comparing.