The paper presents research that aims to expose students' understanding of the "this" reference in object-oriented programming. The study was conducted with high school students (N = 86) and college engineering students (N = 77). Conceptualization of "this" reflects an understanding of objects in general and involves aspects of programming variants and programmers' preferences as well. To examine students' conceptions, perceptions, and misconceptions we developed a diagnostic tool that uses "this" in various contexts, such as in constructors, as a visible parameter, for calling an overloaded constructor in class, or while transiting a non-static method using this to a static one. The detailed analysis revealed difficulties, in both groups of participants, in conceptualizing the meaning of "this" as the current object and in its various uses in the code. The discussion presents students' conceptions of "what is this", nine misconceptions that we characterized, and answers to our research questions. The conclusion offers recommendations for teaching and learning processes in light of the results obtained.