This is an experimental SMSG mathematics text for junior high school students. Key ideas emphasized are structure of arithmetic from an algebraic viewpoint, the real number system as a progressing development, and metric and non-metric relations in geometry. Chapter topics include why study mathematics, decimal and non-decimal numeration, the natural numbers and zero, factoring and primes, divisibility, unsigned rationals, non-metric geometry, measurement, informal geometry, approximation, the lever, statistics, chance, and finite mathematical systems. (MP)