In this study, it was aimed to reveal pre-service middle school mathematics teachers' understandings about the concept of distribution and related concepts. It was adopted, the phenomenographic research method, one of the qualitative research designs, was employed on 66 third-year pre-service teachers. Through the activities developed on the basis of the study conducted by Lee and Meletiou-Mavrotheris (2003), interviews were conducted. The collected data were analyzed within the context of the knowledge and skills intended to be measured. The findings have revealed that the pre-service teachers were more successful in naming histogram axes complying with the context than constructing distribution graphs. They were not able to evaluate real life situations on the basis of the distribution. They had difficulties in establishing connections between the given graphical display of distribution and variation. When they were asked to work on different contexts, it was observed that the pre-service teachers even arrived at correct answers through incorrect reasoning. Thus, it was suggested that pre-service teachers should be engaged in activities helping them recognize the relationships between the concept of distribution and related concepts and should discuss about these concepts.