"Middle school math" refers to the mathematics taught between 4th and 8th grade. In these grades, mathematics education begins the critical work in laying the foundation for success in algebra.

Elementary mathematics in grades K-3 is largely about arithmetic. In these years, math is exploratory in nature, allowing students the opportunity to build up their intuition and number sense as well as their fluency with math facts. 

Middle school math introduces the first of these abstractions: fractions,  rational numbers, and the introduction of unknown quantities ("variables") to students.

While the value of teaching algebra and geometry has been recognized for decades, the value of properly teaching fractions, decimals, percentages, and negative numbers is only now beginning to be recognized. Middle school math is the foundation for success in high school math or beyond. When students lack mastery of middle school math, their future mathematical abilities are compromised. To keep science, technology, and engineering paths open to our students, we must ensure they master middle school math.