Critical incidents are particular events that teachers report as important to their career decisions. This mixed methods study uses critical incidents to analyze how structural factors and teacher agency relate to mathematics teacher retention at the district level. The quantitative analysis of demographic, service history, and survey data identifies structural factors associated with retention. The subsequent, two-part qualitative analysis of teachers' self-reported critical incidents provides insights into the value these teachers place on student relationships, the job of teaching, and professional support. Given their power to explain teachers' retention outcomes, critical incidents merit serious attention by researchers and policymakers alike. (c) 2021 Elsevier Ltd. All rights reserved.