Mathematical writing is essential for students' math learning, but it's often underutilized due to unclear guidelines. Mathematical writing is a mode of communication that provides teachers access to their students' thinking and, importantly, offers students an opportunity to deepen their mathematical understanding, engage in mathematical reasoning, and learn a fundamental way to communicate mathematically. Notably, one needs to be able to judiciously combine mathematical symbols, representations, and text. However, more research is needed to exemplify the qualities of mathematical writing, develop implementation methods, and support teachers. Illuminating and Advancing the Path for Mathematical Writing Research, is a necessary comprehensive resource designed to enhance mathematical writing and promote equitable learning. This research book provides a comprehensive understanding of the current state of mathematical writing and illuminates various perspectives on moving the teaching and learning of k-12 mathematical writing forward. Mathematical writing is an important yet underutilized component of mathematical discourse, and this book offers further insight into understanding what it means to write mathematically for mathematics educators and researchers. It informs with research-based implementation strategies and creates purposeful professional learning opportunities. Ultimately, k-12 students will benefit from a more informed field because they will have access to a vital mode of mathematical reasoning and communication.