The theme of this graduate thesis was to determine the category of grammatical gender and to compare this phenomenon in Polish and Croatian. In the first part two branches of linguistics are mentioned that are involved in gender studies, neurolinguistics and feminist linguistics. The second part is about the semantic (animistic, anthropological, psychological) and grammatical (morphological, morphosyntactic, lexical) theories whose representatives tried to define gender. The third part of the paper talks about the concept of gender and the definitions found in Polish and Croatian sources. Also mentioned are the terms of natural gender and gender identity. The fourth part is about the category of grammatical gender in general and the languages that do not have this category. It describes how the grammatical gender phenomenon is explained by Polish linguists and how it is defined in Polish grammars. Nouns are treated as a primary group possessing the characteristics of the category of gender. Furthermore, the grammatical gender in the Croatian language is examined based on the definitions and information from Croatian grammars. In the fourth part, besides the noun, other types of words from open word classes are included because they also gain the category of gender in a sentence. In addition, the morphological categories of number and grammatical case are discussed, which are present in both languages. The final part discusses the topic of generic names of animals and their grammatical genera in the Polish and Croatian language.