Vocabulary acquisition is one of the most important areas of child language, since it is vocabulary, in particular its size and diversity, that grammar and syntax acquisition depends on. A significant portion of vocabulary is acquired through interaction in a motivating family environment which includes reading materials as well as people who read to children. The major reading materials for children are picture books which represent a significant vocabulary resource. The research presented in this thesis was aimed to determine the size and diversity of vocabulary in the picture books published in Croatia between 2007 and 2009. The corpus consists of the picture books intended for children between the ages of two and three, which is considered to be the period of the most intensive vocabulary acquisition. The theoretical part of the thesis deals with the three periods in the development of research on child language, followed by the different approaches in the study of child language and its acquisition. Furthermore, the thesis discusses vocabulary acquisition under the environmental influence, the parent-child interaction, and the impact of reading on language development, with a particular emphasis on vocabulary development. Special attention is given to picture book with its rich potential to foster vocabulary acquisition. The results of the research are outlined in the second part of the thesis. The compilation of the corpus was based on the analysis of the data on the publishing production as well as on the results of the surveys conducted among the Croatian publishers and librarians. The size and diversity of the vocabulary in picture books and children's vocabulary (based on the transcripts of the Croatian Corpus of Child Language) were analyzed using the software program designed for the analysis of child language CLAN. The results show significant differences in the size and diversity of vocabulary in the picture books published in the period researched. Additionally, there is a huge discrepancy between vocabulary diversity in the picture books and the vocabulary diversity of children for whose age the picture books were intended by both the publishers and the children's librarians. The present research raises many questions for further research and provides guidelines which place the possible implications of the research findings within the realms of linguistics, publishing, and librarianship.