The diploma thesis focuses on generalization tasks. Its aim is to gain insight into the solving strategies that pupils use and to determine whether the chosen approach depends on the acquired knowledge about the variable. The work is divided into theoretical and research part. The theoretical chapter contains three parts. The first is devoted to letters in mathematics and generalization problems as one of the ways of introducing a variable. The next part presents the results of Czech pupils in the international comparative research TIMSS in the problems on generalisation. The last part is devoted to curricular documents and the analysis of selected textbooks in terms of the occurrence of generalization problems. More attention was devoted to the only textbook series which includes generalization tasks. The research part is the core of the work. Its aim is to describe the solution strategies of selected pupils. Four tasks were the basis of the research. They were selected by the author from various research sources and graded according to their difficulty. Pupils who had already encountered the term variable were the target group as well as pupils for whom it was a new term. Eight pupils participated in the research. They were interviewed about their solutions of the assigned tasks. Next, the...