This study explores secondary mathematics teachers' perceptions of the experiences that contributed to their capacities to understand mathematical modeling and to facilitate students' modeling experiences. The retrospective research methods and transformative learning theory frame used in the study honor teachers as adult learners and value their perspectives while providing a way to study the complexity of learning to model and to teach modeling. Data analysis identified triggers and knowledge dilemmas that challenged and prompted teacher learning as well as opportunities to resolve dilemmas through rational discourse and critical reflection. Patterns in teacher-identified meaningful learning experiences reveal a trajectory with strands that address aspects of doing and teaching mathematical modeling: mathematics, social aspects of learning, real-world contexts, student thinking, and curriculum. Results of this study provide a holistic view of learning to do and teach mathematical modeling, complementing studies of designed professional learning interventions that out of necessity target specific parts of the modeling process. The results both support and challenge common teacher education content and practices. The study illustrates the usefulness of retrospective methods to understand teachers as lifelong learners.