The purpose of this study was to investigate middle school students’ understanding of infinity. The sample of the study consisted of 176 students from three middle schools. To explore students’ understanding of infinity, a test including four open-ended questions was administered. Following the test, semi-structured interviews were conducted to produce in-depth data related to students’ understandings of the infinity concept. The data were analyzed using qualitative content analysis method. Results of the study revealed that the participants described the infinity as “having no end”, “continuing” and “never-ending” and perceived infinity “as a process that never ends". The participants’ descriptions were categorized under three different headings, namely mathematical, physical and affective. Besides, it was found that students’ descriptions include static and dynamic notions. It was observed that static infinity notion is common among students.