The bar model diagram is a very important tool that allows students to solve many types of challenging problems. The bar model diagram is actually an algebraic method of using boxes to represent unknown variables. With the model diagram therefore, a student can understand algebraic concepts more visually and concretely. In this book, we explain how to use bar model diagrams to solve problems of varying difficulty in a detailed and simple manner. Thirteen basic techniques of solving problems using model diagrams will be taught. This book is suitable for students from grade four to grade six.