Scaling up mathematical modelling instruction: investigating the impact of an effective professional development program for leading teachers: Hadas completed her master thesis under the supervision of Dr. Zehavit Kohen. The study investigated the professional development (PD) for modelling-based instruction of leading mathematics teachers, while focusing on the implementation strategies that characterize the PD design, and contribute to its scalability. Hadas has a vast experience in teaching mathematics and science subjects in high school classes. She holds a B.Sc from the Faculty of Industrial Engineering and Management at the Technion. She teaches mathematics for gifted students, and owns a private learning center.