The author cites research from students' misconceptions of decimal notation that indicates that many students treat decimals as another whole number to the right of the decimal point. This "whole number thinking" leads some students to believe, in the context of comparing decimals, that "longer is larger" (for example, 0.45 is larger than 0.8 because 0.45 has more digits). The research discussed in this paper indicates the importance of identifying children who develop "whole number" thinking with respect to decimals and to make sure that the learning opportunities provided do not reinforce this misconception. Implications for teachers include: (1) Avoiding rules and tasks that encourage whole number thinking, such as adding zeros to compare unequal length decimals and using equal length decimals in classroom activities; (2) Encouraging use of fractional language to describe decimals; and (3) Using representations such as the "decimat" or linear arithmetic blocks (LAB) in conjunction with comparing, ordering and benchmarking tasks to provide a self checking strategy to solutions. (Contains 9 figures and 1 table.)