One of the fundamental aspects for child's language development is understanding of the read text. Teaching of Croatian language is very important in this context because through it the child learns to read and to understand literary texts, various methodical templates, as well as the required reading list. The understanding of every read text implies that the person who reads it knows the meaning of each word in the text. Teacher is the key person who is supposed to clarify the meaning of each word to their students for complete understanding of the text, but should also insist on these words becoming the active part of the child’s vocabulary. Textbooks most often, along with the texts, contain meanings of unknown words as the part of the methodical instrumentary, and the students, as well as teachers use it so that the students could learn meanings of certain words. Theoretical framework of this thesis work serves as the starting point for the research which provides the review of past research and knowledge regarding this topic. The main aim of the research was to determine to what extent unknown words as the part of the methodical instrumentary differentiate in different textbooks for primary school. By means of a survey questionnaire, the second aim was to investigate different ways in which teachers explain and examine unknown words, but also their opinion on textbooks. The results of the textbook comparison have shown that there are different approaches towards ways of explaining the unknown words, as well as potential problems regarding certain ways of explaining. The results of the survey questionnaire have shown that teachers are mostly satisfied with textbook explanations, but a lot of them think that there is still room for improvement. However, it has also been determined that most of them do not utilize any special ways of examining the comprehension of unknown words, but they consider it is important to know the meaning of unknown words because it contributes to general language competence and it reflects the level of reading literacy in students.