Proportionality can be considered among the most important mathematical notions in the middle school math curriculum (grades 5 to 8). It is the capstone of elementary arithmetic, number, and measurement concepts, and at the same time one of the most elementary understandings one needs for more advanced mathematics. Understanding proportionality is not only essential for comprehending higher level mathematics such as geometrical similarity or probability, it is also most useful for everyday life. The development of proportional reasoning is a complex process that progresses gradually over many years. However, despite the attention teachers pay to this area in the curriculum, students continue to experience many difficulties with it. This research-based practice guide addresses some of the most important difficulties and the ways to deal with them.