The study designed, described, and analyzed the integration of mathematical investigation (MI) and its assessment in secondary mathematics classes. It adopted a qualitative research design in describing and analyzing the nature of the products and processes of MI and in documenting the experiences, constraints, challenges, and changes the teacher and students underwent in the process. Data were collected from two geometry classes and the mathematics teachers of Las Pinas East National High School in Las Pinas City however, data analysis was restricted to 22 students comprising two groups per class. The results of the study provided evidence to the conjecture on the interplay of factors in MI that was proposed in the framework. Further, the results suggested the inclusion of the students metacognitive ability and both the teachers and students communicative competence as factors in MI. To assess MI products and processes, a scoring framework and rubric were developed and validated in this study and a process for assessing MI was proposed. Results indicated that the teachers and students beliefs and attitudes came into play, and were challenged, as they engaged in MIs. Their attitudes towards MI were generally positive. On the average per task, each group of students posed three to four problems, which were generally highly original with moderate to high complexity. The proof schemes used by the students to prove their conjectures, improved with some training from examples-based to transformational in the numeric task. In the non-numeric tasks, students used mostly perceptual and examples-based proof scheme. Finally, the study proposed a design for the initial integration of MI in schools