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Thirty-five teachers teaching advanced topics in high school mathematics were engaged in a year-long intensive professional development project intended to increase their Mathematical Knowledge for Teaching in the area of discrete mathematics. Project activities embodied a models and modeling perspective on pedagogy and learning. Results showed that participating teachers’ content knowledge improved significantly in discrete mathematics, their classroom practice improved significantly. Additionally, participating teachers’ students outperformed their counterparts in a matched-control group in achievement. Results provide evidence of the effectiveness of mathematical modeling for developing teachers’ mathematical knowledge for teaching, and on improving secondary mathematics teachers’ practice.