Developed for elementary teachers, this activity unit is designed to teach students the importance of estimation in developing quantitative thinking. Nine ways in which estimation is useful to students are listed, and five general guidelines are offered to the teacher for planning estimation activities. Specific guidelines are provided for teaching estimation in the following areas: quantity; rounding; addition; subtraction; money; multiplication; division; measurement (including spatial relationship, body measurement, and linear measurement); volume; temperature; time; and mass. The five main activities offered are (1) guessing the number of objects on a piece of wallpaper; (2) weighing rocks; (3) measuring the length of wallpaper, including metric measurement; (4) measuring shoe string licorice using a metric ruler; and (5) estimating the number of chews in a Tootsie Roll, the number of sips in a glass of Kool-Aid, and the amount of time it takes to melt peppermint candy. Eight additional short activities are offered at the end of this unit. Also included is a list of the sources used to develop this unit. (EM)