This paper focuses on the classifications of words which were elaborated in the different schools of ancient Sanskrit grammar (Vyākaraṇa). It describes first the Pāṇinian classification of words, the most elaborate, and then presents the word class system in post-Pāṇinian Sanskrit grammars, which are predominantly topically organized. The final aim of the study is to provide an overview of the word class system in the Sanskrit grammatical tradition, similar to the one given by Robins in 1966 for the European grammatical tradition.