1 Incorporating History in a Mathematics Lesson on Pythagorean Theorem: A Lesson Study Joanne Casanova De La Salle University – Manila <joanne_r_casanova@dlsu.ph> Emilyn Galsote De La Salle University – Manila <ehmzgalsote@gmail.com> Mark Joseph Quinto De La Salle University – Manila <mark_joseph_quinto@dlsu.edu.ph> Mary Grace Escueta De La Salle University – Manila < mary_grace_escueta@dlsu.edu.ph > Napoleon Montero III De La Salle University – Manila <napoleon_monteroiii@dlsu.edu.ph> Levi Esteban Elipane University of the Philippines – Diliman De La Salle University - Manila <levielipane@yahoo.com> INTRODUCTION The Pythagorean Theorem is one of the topics that plays an essential part in many fields of Mathematics. For instance, it is the basis of Trigonometry, and in its arithmetic form, it links Geometry and Algebra (Stone, 2012). Thus, the authors came to agreement to work on the lesson about the Pythagorean Theorem and how to deliver the topic to promote mathematical thinking. It is believed that if the Pythagorean Theorem is taught well, by incorporating with its historical sense, it would be a lot easier for the students to learn the lesson as well as the other fields. This paper intends to provide students a different perspective in terms of learning the lesson on Pythagorean Theorem. Specifically, this study is conducted with the following objectives: (1) to design activities that integrate history of mathematics in the teaching of the Pythagorean Theorem; (2) to analyze how the history of mathematics helps students think critically (3) to examine how the lesson could be improved using the principles of Lesson Study as a tool for evaluating classroom practice. The insights, comments and suggestions about the research lesson and actual teaching demonstration became the fundamental basis of the results discussion and recommendations. Hence, there is a need for a research and development system that would open among its participants a venue for collaboration and exchange of opinions. There is a school-based professional development framework that has been growing more popular not just in Asia, but across the globe. This is called Lesson Study, a framework which originated in Japan. Lesson study: Focus and Features Lesson study, a translation of the Japanese words jugyou (instruction or lesson) and kenkyuu (research or study) is a professional development approach that is continuously growing in popularity among educators nowadays (Lewis, Perry & Murata, 2006).