Integrated or interdisciplinary teaching is an approach that encourages students to synthesize knowledge from more than one discipline in order to solve problems. It focuses on students' interests, a connection of subjects, use of authentic situations and problem solving techniques. This study aimed to make clear the concept of interdisciplinary teaching, by identifying how different subjects could be integrated in a curriculum and how interdisciplinary work could be developed and organized in mathematics classrooms. By reviewing various literature of the research on the topic of interdisciplinary education, the study synthesized ideas, concepts and models in interdisciplinary teaching. Then the study discussed how to apply the explored theory of interdisciplinary teaching in the Vietnamese mathematics situation. It was found out that projects should be used for interdisciplinary mathematics education. A four-step process to design an interdisciplinary project for Vietnamese mathematics classrooms was proposed. This process proved to be a useful guideline for mathematics teachers to design interdisciplinary projects. Two interdisciplinary projects for Mathematics involving Biology as the second subject were introduced in this study. These are examples of how to construct an interdisciplinary project based on the four-step process. Through these projects, students could know how to integrate skills and knowledge of the two subjects Mathematics and Biology to solve a real life problem.