The effectiveness of different concrete and pictorial models on studentsí understanding of the part-whole construct for fractions was investigated. Using interview data from fourth and fifth grade students from three different districts that adopted the Mathematics Trailblazers series, authors identified strengths and limitations of models used. Pattern blocks had limited value in aiding studentsí construction of mental images for the part-whole model as well as limited value in building meaning for fraction addition and subtraction. A paper fraction chart based on paper folding model support studentsí ability to order fractions with the same numerators but were less useful in helping students on estimation tasks. The dot paper model and chips did not support fifth grade studentsí initial understanding of the algorithm.