The fourth edition of *Math Trailblazer s* is compromised of rich problems that are worth solving. These robust problems, combined with careful attention to the progression of activities, reveal what students understand and help teachers analyze developing mathematicians. MTB4 offers opportunities for students to self-evaluate, incorporate prior knowledge, invent and refine strategies, and think about the reasonableness of their answers. A deliberate progression in conceptual development, the purposeful use of representation, explicit instruction about making connections among representations, along with tools that enable teachers to understand how concepts connect to other concepts and how practice connects to other practice exist to develop mathematically proficient students. Watch NCTM’s video Building Conceptual Understanding for Mathematics.