The development of communicative competence is a fundamental goal of Slovene language teaching in Slovene primary schools. As part of this goal, reading literacy is also being developed. In the project Reading Literacy and the Development of Slovene–ARLEM (Awareness, Reading, Language, Evaluation, Models), a reading comprehension test for fifth and eighth grade has been developed, consisting of four tests. This article focuses on two of them, the Vocabulary Test and the Reading Comprehension Test for fifth and eighth grade. A study conducted among more than one thousand students showed a correlation between students’ vocabulary comprehension and reading comprehension, and a relationship between these two variables and teachers’ assessment of students’ reading and students’ grades in the school subjects Slovene, math, and social studies / history. The results also show that, on average, boys score lower than girls, that on average, students whose first language is not Slovene score lower than students whose first language is Slovene, and that students who receive additional professional help with reading score lower than students who do not need additional help with reading. The results confirm the importance of developing vocabulary and reading skills randomly and in a planned way in all school subjects. Furthermore, the results show the importance of acquiring reading techniques over time and developing reading motivation. Furthermore, the results show the need for cross-curricular integration, individualization, and differentiation.