This is a manual for teachers using School Mathematics Study Group (SMSG) primary school text materials. The commentary is organized into three parts. The first part contains an introduction and a section on mathematical comments relating to background knowledge needed by the teacher. The second and main part consists of a chapter by chapter commentary of the text. The objectives, vocabulary, materials, and suggested procedures are given for each lesson. The third part suggests probability devices which might be of use. Chapter topics include: (1) certainty and uncertainty; (2) likelihood; (3) combining events; (4) possibilities; (5) combinations; (6) arrangements; (7) probability; and (8) repeated trials. (MP)