The purpose of this study was to compare the concept images and hierarchical classification skills of students at different educational levels (middle school, high school, and university) regarding parallelograms. In this study, a cross-sectional research design was utilized. The study group consisted of a total of 357 students from middle school (5th-grade, n = 80; 8th-grade, n = 73), high school (9th-grade, n = 49; 12th-grade, n = 49), and university (freshmen preservice mathematics teachers (PMTs), n = 54; senior PMTs, n = 52). Data were collected through a diagnostic test consisting of two separate forms, which required drawing different parallelogram shapes and classifying the parallelograms among the provided shapes. Data were analyzed using descriptive content analysis. It was indicated from the results that most middle and high school students classified the quadrilaterals based on their prototype parallelogram concept images, and some also had missing definitions as well as misconceptions regarding parallelograms. Additionally, it was revealed that although the majority of pre-service teachers classified the parallelograms hierarchically, some were deficient in their concept images of parallelograms.